I actually don't "know" either. But I suspect it has to do with knowing which python to use (system generally) and if the user wants to use global packages as well.

checkout virtualenvwrapper (http://virtualenvwrapper.readthedocs.org/en/latest/install.h...) as well for handling virtualenvs.

Honestly, I have no idea. I haven't ever tried to move a virtualenv, and it's not something that's supposed to be part of your repo (you should build your env separately on every box; it's intended to encapsulate dependencies from the system, not to transfer them around with your code).

If your system gets updated/upgraded to the point where your virtualenv breaks, rebuild it from scratch; all it should take is a bit of time.

[edit: I am not a developer involved with virtualenv, and the statement about what it's intended for probably shouldn't be quite so strong, as this is only my impression of it and the way I use it.]