Personally I don't attend to many (maybe because there are not many where I live) but I've attended to a few. I've been professionally coding for 5 years so I don't consider myself a senior but here is what I've learned through these events.
Attending a hackathon didn't make me better at programming (perhaps a bit), but it strengthen other important skills that helped me like networking and team work under fast environments which are very important in any career. You will meet a lot of like minded people so talk to them, maybe you could find a mentor or a good friend with great skills.
Also, there is a lot of tech hype around these events so you will get to experience how people solve problems in different ways, this gave me a notion of what new technologies are being used and inspired me to continue learning.
So in general, your programming skills may or may not improve but the biggest gain (IMO) is the people you meet, the soft skills you put in practice and the inspiration.