Most people are just too lazy to scrape it off the back. That is certaintly true in my case. Some people feel it looks more natural, and some fish such as african cichlids, mollies, and plecos to name a few, forage on algae in nature, so in the tank it works as a filler, and gives them something to do. There aren't any negative impacts to removing it, just the elbow grease and time to do it.