Because some of us are so fat/strong that we can't get our cogs off after riding for a few months. I broke my chainwhip (Park, not some cheapie) and the owner of my LBS bent the handle of his trying to get a cog off one of my wheels. I tried rotafixa, too, but it felt like spokes would start breaking before the cog came loose (not that they would, just that that's what it felt like), so I gave up. And, before you say anything, I greased the **** out of the threads before I installed it.