I wrote my first game for #sadmechjam, and just finished my second for #WizJam2k19. As someone who has a serious tendency to let projects grow to nigh unmanageable proportions (I'm looking at you, hundreds of pages of comics), I find game-jams - with clear themes, definite deadlines and a solid goal - a good way for me to get things finished.
I've got another game that I've been tinkering with for a while, but for now it's mostly a big stack of notes, brainstorming documents and a general goal... and one of several reasons why I haven't gotten it into a playable shape yet is that there isn't any external pressure to do so. Getting the chance to write and actually finish games has been healthy for me.
Also, themed jams gives you the chance to maybe step outside your comfortable box occasionally. Genre-wise - both as a player and a creator - I feel very at home in the fantasy/magic bullshit subgenre... but the #sadmechjam let me play in the mecha sandbox for a bit, for example. It pushes me creatively in a way that I enjoy.