PDC are primarily for anti-missile defense, but can be used in close quarters. They are mostly useless for destroying or disabling a ship, though they can cause damage to the crew.
Missiles and other explosive ordinance aren’t exactly projectiles, in that they are pretty useless as projectiles alone. Disable a missile’s warhead and it just bounces off or lodges in a ship’s armor. This was shown first-hand in S6 when Fillip digs one out of the Pella’s hull.
