It also comes in handy when I'm trying to install something finicky that has a million dependencies, each of which also might be finicky (e.g. numba) and I screw something up and just want to start over clean and not muck around uninstalling things. virtualenv makes this easy—I just make a new environment for experimenting and then delete it if I mess up and want to start over, leaving my system configuration clean and other environments intact.