In short you change the effect of incoming attacks or spells less than one time in twenty. It's nice but I wouldn't go so far as to say "A lot of importance".
IYou will generally reduce the damage you take between 30% and 10% depending on your base AC and the higher save DC from your 18 will only give you one more spell prepared and only change the save on one out of every 20 spells you cast.
The main one of those that should be forcing save DCs regularly is the Monk (Stunning Fist). The Rune Knight has only two runes that inflict saving throws, meaning that on average one roll gets its result changed at most once every ten rests on average if you have a 16 rather than 14 Con, take all the saving throw options you can, and fight all out every rest. Literally the only saving throw spell I've seen a paladin use is Compelled Duel (I'm not calling Blinding Smite bad - just I've never seen it). Arcane Tricksters are similar; a good illusion doesn't provoke a save.
At level 7+ Fey Wanderer Rangers force more saves than any other class I have played, including full wizards, sorcerers and Monks.
Many of the Paladin Channel Divinity options require saves, some of them cause saves turn after turn and others cause saves for multiple enemies.
A good illusion does not provoke a save but Tashas laughter, which has been on every AT Ive seen played, does require a save. So does suggestion, and Tasha's mind whip and on of those is on every AT of 7+ level I have seen played.
Again we're talking about an effective change of one roll in 20 on something that you don't roll often.
And with a Paladin who gets fireballed once with 3 allies around him that is 4 rolls on one single turn. Skills like Perception, Persuasion and Stealth are used a lot. At low levels (when the 18 really matters) those skills are used MORE than spells are.
The reason your primary stat is so important is that you are going to be rolling against it or forcing other people to roll against it almost every turn, sometimes multiple times. And it's something you choose. Again a second 16 at a cost elsewhere is nice but I wouldn't go so far as to say "A lot of importance".
No you won't. That is just untrue. It may be true for some builds, but not all or even most. The Cleric I mention above who is a custom race uses Strength FAR, FAR more than wisdom. Like 4:1 as much.
In the 2 last game sessions (2 adventuring days) we have been in 9 fights. In combat during that time she cast Fog Cloud once (ability irrelevant), Misty Step twice (ability irrelevant), Shield of Faith once (ability irrelevant, Toll the dead once (ability mattered), Destructive Wrath 4 times (ability mattered) and cast Dissonent Whispers once (ability mattered).
So there is a total of 6 times where she used her wisdom and four of those were reactions (DW) not actions. So in 8 entire combats she only used Wisdom as an action against an enemy 2 times on her turn (the toll the dead and the DW)
Counterbalancing that she used shove twice (one to shove an enemy down a well and once to shove an enemy off a boat)
Every other action she made during those combats (and several reactions) was an attack using strength. She used a whip for one attack, threw a Javelin for one and used her warhammer for the rest.
I don't think that is uncommon. My Bladesingers all used Dexterity for attacking more than intelligence for attacking or saving in tier 1 and 2. The only sorcerer I played used Dexterity more than Charisma all the way through level 4 and even after level 4 it was probably close until level 6 or 7. Druids are going to be doing things in wild shape more than they are casting offensive spells until level 8 at which point they should have a 20 anyway.
And again this is before you consider multiclassing and in any discussion of how broken a race is, we would need to consider how broken some multiclass combos can be and it is a lot easier to do if you can start with 16s in 2 or 3 stats. A Bladesinger/Hexblade is the most powerful character I have seen played and to do it you want a half elf that starts with three 16s. At level 12/4 you can attack with your crossbow and shoot three eldritch blasts getting a +5 hit and damage on all 4 of those attacks. You can tack on 20 more with Hexblade curse and whenever you are not using a bonus action you have a Sprite shooting poison arrows with a 18DC. You are also a full 12th level Wizard with a 18DC save on your wizard spells and a 26AC in bladesong with shield. You can do all that with a PHB Half-Elf.
