Backgrounds don't try to explain your class mechanics. Whether you're a Noble or a Hermit, neither of those try and explain where your Paladin or Wizard powers come from. They exist to add depth to a character, instead of anyone just being their class. They don't change the lore behind the class mechanics, though.
True they don't expand your class but they do expand your character, which supports "you class is not your character" since if "your class is your character" you should get nothing from your background. You do get skills, tool proficiencies, and a special feature that informs your character regardless of your class. I will admit, sometimes a player has little more than bonus skills, but other times it completely changes the personality and play style of a character. A good example might be my human variant
Urchin The Great Old One Patron Pact of the Tome Warlock scout. He and Observant from human variant, stealth and thieves tools from Urchin and was essentially the party Scout sniper with Devil's sight and Agonizing Blast. He hide and sniped like a ranger and scouting, searching for traps, and unlocking doors like a rogue (Had gloves of thievery). 100% pure warlock so it was not a munchkin and when we built the party we choose roles not classes and I choose party scout. That was my job in the party so I was stepping on no ones toes from session 1 to the end of the campaign. I simply chose to do it in a different way, which actually worked very well. Darkness + Devil's sight while in the dark is kind of like casting invisibility that doesn't fail if you attack. I also means I was completely obscured so I could hide literally anywhere, but I didn't even need to do that to have advantage on attacks from being unseen and enemies attacks on me at disadvantage for blind firing at a heavily obscured opponent.
The cool thing about The Great Old One patron is that my GM at the time was a "
You class is your character" GM for warlock and cleric (and nothing else), so he originally tried to tell me I could not play a non-evil warlock then I showed him this, "
The Great Old One might be unaware of your existence or entirely indifferent to you, but the secrets you have learned allow you to draw your magic from it." So as I said before you have to form some level of agreement with your GM if the two of you disagree about "
You class is your character" or "
You class is not your character", I did this by excepting my GMs stance and picking a sub-class that specifically says that is ok. This still came up around level 8 because despite my background explaining how I got my pact without the Great Old One knowing about me, My GM made my patron become aware of me then tried to force a conflict between my patron and the Deity of the Party Paladin. I sided with the Paladin which surprised and Angered the GM, not because of PHB fluff or because it didn't fit the story (my character was established as the very loyal second in command to the party leader/paladin due to mutual suffering at the hands of the same cult that started both our paths). No, he was made because it was not how he would have played it or wanted me to play it. He had agreed to my character at level one, after me showing the PHB fluff but as it turned out his "
You class is your character" stance on warlocks extends to Celestial Warlocks being evil because they turned to a creature for power, which says to me "
You class is your character" really means "
I have this personal stance on what a class is in D&D and I want to enforce my stance on your character design and play using "fluff" I create and if you have "fluff" that counters it I will still change the world to make you wrong and push you into playing the way I want because I am GM and my desire is your law" …. I played a that table a while more but this kept coming up. I would create a character, he would approve it, then when I played it he didn't like how, then he would alter the world and claim innate fluff to force me to play my character a specific way that suited him. After a few short campaigns, I told him "I am not an NPC" to which he basically said "yes you are! you all are! this is my world! My story! In my world my story is the only thing that matters! I just let you tag along! Play your part!" … I left the group after that.
As a result of this I have learned 3 things.
1. Fluff is more often an excuse for a GM to control player style than not.
That is not always the case but it often
seems that way to the player, true or not. So anytime I see a GM pushing fluff, I can't say they are wrong and I recommend players and GM working it out between them. I also, try to keep this in mind when I GM, so that I don't use fluff to attempt to trap my player into my personal feelings even if its subconscious. (That doesn't ignore all since, it means the player needs to work on background that brings something around that I want to no to, bringing it fare enough I can bare to say yes to it. I will then hold them to that background)
2. You know if the GM is "
You class is your character" or not if they are presented with fluff from the rules they argue your going against the proves they are wrong or that you can make a character that does not have the quality they are saying you must have, then alters their setting/story to make themselves right, this is not a debate about fluff.
You just have a Narrator instead of a GM. If your GM constantly says "story comes first" that is also a good indicator of this. Run from this table if you don't want to be an NPC in there story. When I GM,
I try to remember the concept of role playing games is a shared story. To me that means, the story a player wants to tell is just as important as what the GM wants to tell. The GMs job is not to narrate, but mediate the rules and give the game direction through story hooks when players don't have a direction. If a player drops a story hook that players or the GM want to grab, that's is completely expectable. My current GM is really good at that. I need work on it, since I get a bit frantic and cough up in keeping track of things when I run games. So I try to listen more to the players when its not "my turn" the way I expect them to listen to me when I am describing a scene or feeding a story arch. I also try to listen to feedback desires in conversations between sessions.
3.
I am jaded about being controlled like an NPC as player and so I don't want to do this to my players. If the book says I can choose and option that my patron is not a nuisance and a GM tries to force Patron nuisance on me despite my background, player choices of subclass, and conversations explaining what I am trying to avoid, then I am done at their table. At the same time, that means reading players back grounds and knowing that sometimes backgrounds are empty because players don't want to look back but move forward. Sometimes they clearly cut of allies and hooks and I need to respect that the way I want it respected when I play. Sometimes a player who has played a warlock leave a juicy hook to their patron in their back ground so I as GM will run that story while other times they have done that and they just want to be a warlock class without the patron drama so they have fulfilled their pact with their patron before the game started.