Career knights endure many campaigns, not all of which are well supplied. Sometimes, combat comes down to fighting with whatever makeshift weapons are on hand, so grizzled veterans come to feel at home with any instrument of violence. In extreme cases, veteran knights have even been known to resort to touch spells.
Weapon masters have improved chance to hit with all weapons and touch spells. The smarter the knight, the better the acquired career skills.
It is the opposite of new Tumble, adding up to 20% directly to your chance to hit (10% from ability, 10% from stat, which is Intellect). In other words, Weapon Mastery can completely negate Tumble. Against players and monsters that don't have Tumble, the weaponmaster's chance to hit rises dramatically. Assuming 100 Intellect, a weaponmaster's minimum chance to hit is 32.5% (base 12.5% + 20% from Weapon Mastery). You could, for example, be a 0 aim garbage character with no weapon skills, and you'd still have a roughly 1/3rd chance to hit. If you have very high Offense, you can guarantee a hit now, when before the defender would always have a minimum 12.5% chance to evade. Weapon Mastery works with any weapon and even touch spells, but not punch.