In spite of its description, the Grand Mace actually does Blunt/Pierce damage and is currently the only 2handed weapon to do so. This damage typing really defines the weapon as a whole, making it more suitable than the Maul against Vanguards but less suitable against other Knights. It's a slow but fairly user-friendly weapon. The Grand Mace recommended in more situations than the Maul in general play at the cost of the (occasionally vital) extra damage against Knights. Arguably, the Blunt/pierce damage typing also makes it very ineffective against archers, but the Grand Mace's raw damage is more than enough to compensate.
The overhead swing is often recommended to take advantage of the weapon's high damage output, alongside traditional horizontal swings. In contrast, the Grand Mace's low damage stabs should be reserved for finishing off foes or catching foes off guard.