Yes, in short, for the money. But it's more than that, for me personally it's because I don't want a 9-5 desk job, doing some meaningless paperwork for the next 35-40 years, and hopefully will be able to contribute towards a greater understanding of (some aspect of) humanity.
I was told by someone at work the other day (an accountant), that I was just going back to university because I didn't want a proper job! How cheeky! I'll be glad in a few years time when (hopefully) I'm paid for doing something I genuinely enjoy rather than just a job that pays the bill.
I hate having to wait until 6PM to get the post…but then again, I'd probably hate it equally if I was constantly peeking outside the window, stalking the postman everyday...