Skeleton Mastery is a skill that passively improves important statistics of all non-golem summons the Necromancer has. It is essential if the Necromancer intends to focus on summoning.
Improved Missile Damage past Level 2
As the Necromancer's knowledge of the undead improves, he is able to harness and divert more powerful spirits to inhabit the corpses of the recently dead. A Necromancer who possesses this skill is able to raise skeletons and skeletal mages of a much more powerful variety.
↑Diablo II Manual, page 75. Blizzard Entertainment, 2000.