Just because you don't find it useful doesn't mean that NO-ONE else will find it useful. For instance, someone might want it because they use the prefer mouse over the keyboard and like to have all their commonly accessed things in the menubar.

I personally use it because i find i often have my hand on the trackpad and not the keyboard when i want to access expose. I would use the corners but i find myself accidentally moving the mouse in to the corners and accidentally activating expose. This is just an alternative to the ways apple offer.

$6 really isn't very expensive for an app that some people might use every day.