Why do fry cooks use water to cook burgers?

I'm sitting at a diner and I'm curious if there is a reason as to why the fry cook cooked my burger the way he did.

The process is different from the one I'd use at home. The cook put the burger on the flat top, the covered it with a metal domed item that looks like a cloche, the cook used a squirt bottle of water to squirt water onto the flat top and covered the burger and the now-steaming water. I've seen this done to quicker melting cheese onto the tops of burgers, but it's never something I've done at home.

Why is this the process that seemingly every diner cooks with? Why is it not commonly done at home or am I wrong, and it is?