In my playgroup. most people play characters with at least some magic. Not everyone, not all the time- sometimes people are happy to play simpler characters where less thought is required. There's certainly a lot of visceral fun to be had in just throwing dice and imagining yourself slaughtering foes with a big sword, for example.
I recently polled the players in my campaign, asking them questions about not only my game but what they found enjoyable/frustrating. One player straight up admitted they don't like the complexity of casters, but they find themselves playing one all the same. They explained that when they played, say, a Fighter or a Barbarian, that they didn't have enough answers in their toolbox. It's like the old adage, when all you have is a hammer, everything looks like a nail.
But in this case, they often found themselves wishing they had more tools to interact with the game. Melee in particular can be brutal in D&D. Enemies can deal lots of damage, they might have riders on their attacks, special traits like auras, or reactions that can completely wreck their character.
Like, say, for example, something like a Chuul, which hits you, automatically grapples you (requiring you to use a full action to break free) and, if it so chooses, can also poison and also paralyze you so you can't even act, and the only thing you can do about is roll a decent saving throw.
But characters with spells might be able to Shield or Silvery Barbs the initial attack so it doesn't hit. Or cast a spell that increases their AC or imposes disadvantage on attacks to help them survive getting in melee. Maybe they cast a spell to boost their saving throws. And if they do get grappled, maybe they can Misty Step as a bonus action to get away without having to make an ability check and still have their Action to use.
And sure, all that stuff is limited use, but it's a resource their class just doesn't have. It's nice to be able to have an active defense in your back pocket, and so they decide they'd rather have such things than not, so they end up playing casters.
Now it's not like non-magic characters have nothing- you might have great ability checks to break free and great Con saves. Maybe your class lets you freely Withdraw or Dodge so you can weave in and out of combat. Maybe you can reduce incoming damage, gain an extra action, or have a little self healing. At high levels you might have the ability to just make a saving throw a few times per day.
But none of this lets you say "you know what? I just don't want to deal with this sort of thing today", which is something magic can let you accomplish. I started playing again recently, and just last session, I survived a Roper because I had the foresight to cast Blink. And yeah, that cost a precious spell slot, and I had to get lucky, but I was able to nope out of a bad situation. Something I wouldn't have been able to do as a Fighter or Barbarian.
Does it make my choice of class superior? Not necessarily, but it sure is nice having options. Like I said in my previous post, it's about potential power.
Now on the flip side, caster-heavy games have their own problems at lower levels. Enemies don't die as quickly as they need to, because you don't have sustained damage. You might lock down an enemy for a round or two, but you're just delaying a problem that isn't going anywhere! So ideally, you want a balanced group.
At least until the party starts using spell combos. That's enough to trigger my DM PTSD right there, when two players ask each other "Say, what do you suppose happens if I cast Sleet Storm over the area of your Evard's Black Tentacles?" and the Druid chimes in "I can cast Entangle if you want!" (Shudders).
