Traffic avoidance. In the SFO area, it's always been a great service and they're used to lots of aircraft doing it. All the NorCal controllers I've ever talked to agree that if you're VFR around the Bravo and Charlie, even avoiding it, they'd rather talk to you. I've never felt like ATC out here has ever been paying no attention and letting planes wizz past me, they're very proactive and always coordinate whatever VFR pilots ask for unless SFO has closed off the surface to transitions or there is a TFR. Even when I go out to Sacramento, Chico, down to Monterey, etc they're also very good about it.
LOL, a buddy at NorCal told me he always does this when flying GA. If SFO says no surface transitions and he's coming up the Peninsula, he just heads to another sector to come in from another angle and ends up being cleared through at a higher altitude than normal. I also request flight following on my initial contact with ground, never heard a peep about that annoying controllers during my various tower tours and I hear others do the same frequency.
All that said, I have noticed that the West Coast controllers are really chill compared to a lot of the country. As Air Wagner has shown us, my experience is you have a 99% chance of getting, "Hey, don't do that" or "Uh, you're in the Bravo get out" and not a phone number so long as you aren't screwing up left and right or giving them any attitude, or of course causing a loss of seperation. Oakland Center and most towers also tend to be comparatively laid back, even in busy areas. People from other parts of the country make it sound like life is so terrible here, yet, people seem chill and happy.