First, you need to take care of the virtualenv manually. This tool avoid that. You can completly ignore the virtualenv. You don't even have to populate requirements or learn about pip freeze.

Secondly, this tool allow you to freeze your requirement list at specific versions. So in your req file, you have the name of the packages you depend on. Bon on the req lock, you get all the pulled dependencies recursively with the version you are using right now. The first one let you dev more easily. The second one deploy more easily.

All that, for less complexity. Win win.