If you want that servo will go to one position for example in the middle, you have two choices:
Enable pwm all the time.
Or enable pwm just little amount of time that servo can turn i this possition.
The main differences between them are:
if you doing first option, servo won't let you turn it by your hand or something, i mean i holds this position and dont change it until you give other duty cycle. The second option doesnt do this. It turns servo to position, but you can easily change it with mechanical energy.
P.S sorry for bad eng. i hope you understood me.