Because you are new to Python
If you want to get better at Python then Pycon is one of the better avenues to learn. Think of it as a nine day data dump of Pythonic knowledge straight to your brain!
When it comes to the fundamentals of Python, the tutorials are a great way to hit the ground running. How do you think I learned? Also, there are dozens of beginner talks designed to introduce you what highly qualified people think you should be doing. Finally, the sprints are a great way to work with real experts for days - because even the most advanced projects needs people to knock out the invariable score of simple bugs, do documentation and write additional tests.
Also, Zed Shaw is going to teach his Learning Python the Hard Way tutorial throughout the conference. (follow that link!) To summarize, besides going over his lessons and some basic conference coaching, he's going to take his class to conference talks and then bring them back to apply what was learned. He won't be doing it alone either - a number of other python folk are going to volunteer their time to help, and I'm going to be one of them.
Because you are old to Python
A good chunk of the tutorials are designed specifically for experienced Pythonistas wishing to expand their knowledge (OMG Raymond Hettinger). And then as we all know the conference itself has scores of advanced talks designed to challenge you. Finally, what better place to work on your favorite project or get people to work on your project then the massive Pycon sprints ?
Want to do Python related technology full-time? What better place to look then where amazing companies and organizations hang out with the intention of hiring people? And invariably some of those hiring groups will be hosting parties too...
Because you are recruiting
Looking for good staff to do Python, Django, Plone, Pyramid, wxPython, NoSQL, Linux admin, Unix admin, BSD admin, Windows admin, Science, or Engineering work? Pycon will be where 1500 of the best and brightest people on the planet will hang out for over a week and some of them will be interested in new opportunities. Sponsorship is a great way to get yourself noticed by hundreds of Pythonistas.
Because it is cheap
Until January 25th Pycon registration is at its early bird prices. Even when it goes up it is still a pretty good deal. Admittedly the biggest expense is the hotel ($159/night) but there are decent and much cheaper hotels ($80/night last I checked) within a few blocks of the conference.