Oral sex can also be a lot more relaxed and prolonged. It doesn't take as much physical exertion to get into position. Lubrication isn't a problem. Erection isn't as much of a problem.
Maintaining penetration isn't as difficult, or as important. If the mouth slips off the target, it's not difficult to re-engage.
You have a lot more control of hard/soft, fast/slow, etc.
If you're going with BBBJ and CFS, you can start and stop BBBJ half a dozen times without having to worry about another cover. Or getting junior to stand up and go back in again.