It's definitely about personalization and status in the community. Some skins are super rare and expensive so having them is like a flex. When CS2 launched there were changes to how the trading system worked which I read about at https://davidicke.com/2023/06/27/trading-skins-in-counter-strike-2-what-changes/ and it shifted a lot of prices around. For some players collecting and trading skins is almost as fun as the actual game. Others just like making their weapons look cool. It's optional but adds another dimension to the experience beyond just shooting