It's a crew call button, you can use it to call the crew. But, yes, I'd say the crew isn't there for our every beck-and-call and if everyone decided to press the call button, they'd be up and down the aisle constantly. For me, it's about having a little respect for fellow human's trying to offer us a service and what I'd expect if I was in that role. ie. pressing a call button for a top-up, really?

I've been told that if someone repeat-presses the button for a top-up it will start being ignored or the crew reaction will be lowered.

Of course, if you never see crew and they never offer top-up, which can and does happen, then it's fair to make the occasional crew call. But using it just because it's there is a different matter, personally speaking. I simply try and catch their eye when they are walking past - just seems more human and respectful.