It mostly kicks in when you are a developer that supports 4 or 5 different ruby-on-rails sites, each of which require different versions of ruby. Rails, in particular, seems to be very intolerant of running on different ruby versions, so if you have been less than scrupulous in keeping all your rails apps up to date, you will be juggling different ruby versions. Using Debian "alternatives" is one way of dealing with it, but you also need to be hyper sensitive about the gem versions, so you will be using bundler in most cases. Rbenv, et all, just deals with the complexity so that it is hard to make a mistake. It's not impossible to do it other ways, but it is much harder.