I think they're used here on the boards as a form of emphasis. Rather than having to take your hands off they key board and reaching for that BOLD icon, people put in quotes b/c they can just type them in and not have to do any "extra work" (see -- I didn't have to use my mouse and stop my typing for that!). Know what I mean?
I have used the quotes, but have recently reverted to using the * for emphasis. Again, I'm lazy and want to get my thoughts down and don't want to lift my fingers from the keyboard to use the mouse until I'm done typing my post to finally go and post it.