As a freelancer you go to a conference to meet customers and colleagues and remember them that you're still alive :-) and still working on the technologies the conference is about. The best way to signal that is to give speech.
As an employee:
If you are in the position to spend money, hire people, buy services, you also meet people you work with and it helps.
If you're junior in general or in some technology, it can be a way to be exposed to it and learn some unknown unknown.
Anything in between, maybe it's just a nice day off, paid by the company.
But there are more killer apps per industry than there are fingers to count with, with more relevant use cases being discovered practically daily. Yesterday was better, but now is when you make every effort to figure out what you're doing to stay relevant before someone else can tell you that you're not.
As an employee:
If you are in the position to spend money, hire people, buy services, you also meet people you work with and it helps.
If you're junior in general or in some technology, it can be a way to be exposed to it and learn some unknown unknown.
Anything in between, maybe it's just a nice day off, paid by the company.