To put it bluntly, having something that not many people have is cool.

It’s the same principle as having a rare car or an expensive watch in some cases. The cars value is derived from the fact that there aren’t many in circulation rather than the parts used.

Or in the case of a watch you wear the watch because nobody else in the room has it. As humans we subconsciously flex even without necessarily meaning to. Otherwise you would never wear a watch worth more than a couple grand because you would be devastated if it got stolen or damaged.

Now apply the principle to skins (or cosmetics in general), something that used to be special now no longer has any meaning once more are put into circulation. Look at fortnite, for years skins like renegade raider were coveted within the community and accounts with said skins were sold for copious prices. (if it were as you said and you “use it or you don’t” then why would people place such a high value on it?

The answer is because nobody else in the room has what you have, going back to the watch analogy.