Pure flavour!

Blizzard wated the races to ahve certain feels to them, certainyl there are laore reasons in soemc ases but, for example, there is no reason that a Belf warrior can't exist. In fact many do, in the game, you just can't play as one. Human Hunters too,etc.

You'll find this is the case for many of the more believable class/race combinations that aren't actually available to be played. They can/do exist we're just simply not able to play as them.

Think of it like this, when you look at a Belf in game, you know what classes it can be, essentially, it'll be a mana user.(If it's your enemy your not likely to see a rogue coming if they're any good) this is very much the flavor Blizz wanted to give player Belfs. At the end of the day we know a Belf is more than likely going to be a ranged DPS unless they're in plate. The same philosophy extends to the other races/classes where the "lore" doesn't negate reason. It's like we're filling the role for how Blizz thought the races would be populated in the "real" azeroth.

Also it's another reason that gives racial selections at character selection a reason for existing beyond aesthetics.