What are garnishes for?

I’m a recovering alcoholic, but I’ve never set foot in a bar, haha. So I have no idea what garnighes are for. I understand an orange peel inside your drink for taste. But a whole cherry in your drink or two olives on a stick..? That doesn’t affect taste does it?

Don’t worry about setting me back. The stuff doesn’t interest me one bit anymore but I see this in shows and movies and I got curious. 🙂