I always wondered about this... I always disable email notifications except for PMs and secret mod stuffs... Are you saying that you wouldn't come here unless you got an email?
Sometimes I'm participating in multiple threads at once and I can't remember the names of all of them, so getting those e-mail notifications is extremely helpful in staying current with those topics. It's more efficient (faster) than scanning the whole board, plus I may not remember where I have or haven't posted recently.
This is indeed the way I come into MSFN most often. There are times when I do visit just to see what's new, but I'd estimate that
80 90% of the time I come in it's in response to an e-mail notification telling me that someone on a topic I'm following has posted a reply here. There's a lot of stuff going on out there in life and many different things vying for one's attention (spouse, work, family, TV, ...), so it helps a lot to get those notices.