I don't think of Java, Win 7, and Virtualization as "apps". I think of them as tools to get something else done. In fact, I think of MS Office as a set of tools to get other things done.
So, when you ask "what for", my context is entirely different than most of the answers so far.
The best way I can answer "what for" is this: a lot of communicating, a little bit of control, and a tiny bit of data management. I just finished purging all the relevant photos off my oldest computer. Now I can reinstall the original factory software, and offer it on Freecycle.
If somebody takes me up on it, I'll get the warm glow of having done some good. I'll also get an empty desktop. At that point, I'll think about what I want to put on that desk.