The purpose is to cause turnovers. The drop-kicked ball behaves a little differently to a tee-kicked ball, we're kicking to guys not used to catching kicks, and the drop-kick gives more hang time so our guys can be there.
As we refine the technique, I would like us to kick the ball off to around the 30 or 35 so that we can have our guys there to compete for the ball - keep in mind it's a live football.
You give up 10 or so yards versus a touchback, but you gain an opportunity to recover the football, which you don't have on the touchback.
If you have difficulty with the concept, try watching some rugby games and it should soon be clear.