Yeah they're just visual but that's part of the fun for a lot of players. Some skins are rare and become valuable which is why trading exists. If you want to learn about different skins and what makes them worth money you can check here for detailed explanations. It covers things like wear conditions, patterns, and which items are popular in the market right now. I didn't care about skins at first but after playing for a while I wanted my weapons to look unique. Now I trade occasionally and it's become its own mini game. Just don't spend more than you're comfortable with because prices fluctuate constantly