They're a break from the "norm".
There are countless networking upportunities, both "official" and "unofficial".
You will reconnect with people you may have lost touch with.
You will make new friends.
You have the opportunity to help others.
You can find out ahead of time if your suit works or not.
It will force you to keep your info/ resume up to date.
You can practice your interview skills.
You can help support an organization you believe in.
And, MAYBE, it will result in getting the call...
I personally try not to look at it like I'm spending time/ money with the expectation of directly influencing my career progression. That will drive you nuts! I've been to four...
Or, don't go. Less competition for me.