Wild ass guess here that you're talking about the servo knob example since you didn't say?
delay(15); // waits for the servo to get there
As it says in the comment it's to give the servo time to get to the new position. Without that perhaps it's possible that before it got there, you would command it to start coming back again. That may or may not be the case for you depending on the servo speed, nor might it matter.
I'd say if it makes no difference to your servo and your requirements, leave it out.