Your AM may be full once the planet is destroyed, and then again it may not.
It depends if you took the time to clear the gravity well before bombing or if you've snuck some siege frigates in, and possibly a Revelation, and bombed the planet while in combat.
There you go, 75% increased AM regen for the rest of the battle.
Even if you've killed all enemy ships and structures before bombing the planet, the bonus lasts long enough that it will still be there for your next battle, unless you sit idle and do nothing.
Basically it's to give the Advent Rebels some momentum. If you let them get one planet, they'll keep going until you crush their fleet because their shields and hulls will be getting replenished very fast and AM will not be running out so they can go from the first planet to a whole series of planets non-stop.