Learning and actually completing something. I started developing my first game nearly 4 years ago and still haven't finished a single game. Most of that time I've also spent on a single large solo project, so my skillset is quite narrow. Still want to get into the game industry and got a few months mostly free, so I figured it a good time to try and build up a portfolio, and game jams provide a good framework/incentive for it.
Not that I actually managed to finish my game in either of the previous two jams I joined, but just gotta keep trying.