Because sometimes one needs to make subtle changes when the bite shuts off on a certain productive method. I used to do just fine with a plain gold and silver. Then I noticed that the hammered pattern picked up some bites when I couldn't get a bite. Then also noticed that during low light, glows get more solid bites. Noticed that when fishing deeper water, the UV ones got a more solid bite. Not always as such the case but sometimes just a little difference.
Keep this in mind though. The old fluorescent orange and chartreuse paint were naturally UV reflective. So I don't just went all hyped over buying variety of UV lures.