It's a simple and the most fair model we have to date. Valve pioneered it with TF2 and perfected it later with CS:GO and Dota 2. 

Give users a complete functional experience for free, and sell them emotional experiences - hats, knives, etc. #nostrdesign 

 skins provide stimulation even when the game itself doesn't so much anymore

there's also the status and hierarchy aspect involved. those with higher buying power can flex their muscles in-game, while it has zero effect on gameplay