I think a lot of it is durablility, sensitivity, and stength.
If you bash your buggy rig than 2 days a week stock servo's just don't hold up well. They also have the strength to move your tires sitting still or under power, you don't have to be moving to turn the tires.
Sensitivity os big when you are trying to fight your way to first place, and you have to manuever in between your competitiors. If you twitch the whel or your transmittier your tires are going to twitch too, unlike most stock servos that seem to slam to one side or the other, or sit an jerk when asked to move minute distances. It really does make a difference when your trying to get setup for a jump and the tie before you get there is limited..
That being said, I use an overvoltage RX pack that makes my stock servos have more power, not being a racer i'll just suffer until they strip out or burn out. I've run high end servos and they are worth it I just have three kids and three nitros that have to run at any given time (budget)