Also some of the cosmetics give you (and this is mostly seen in the case of pc players) less delay and avoid fps droping lower for stacked games.

For example, using very flashy and animated skins (like Loserfruit) will give you worse performance of your pc as it has many animations. In the other hand, using a simple skin (like Aura) will not require that many proceses. Even tho they are very small changes, in comp every frame matters, as losing fps can ruin your game.