As a developer who also programs as a hobby, I dislike having to manually download, install, and configure each framework/platform I want to experiment with. With cloud IDEs like Cloud9, everything is already configured for me and just works.

For my side projects I find this workflow particularly awesome: pull project from GitHub, program in a C9 workspace, and push to Heroku when ready. My development environment is not limited to a physical workstation, and no time is wasted hassling over configuration issues.