Bushido drives the abilities that make a samurai or sampire viable solo: Confidence for the heal-on-hit, Evasion for the defensive window, and Lightning Strike and Momentum Strike for damage. Every one of them gets more reliable and more effective as the skill goes up, and each fails outright often enough at 100 to matter in a long fight.
Bushido also contributes to how well you parry with a weapon rather than a shield, which is the reason sampires run it alongside Parry instead of choosing between them.
This legendary powerscroll raises the cap from 100 to 120 for a single character and is consumed when read. It is one of the cheaper 120s relative to how much of a build depends on it, which is why it is usually the first scroll people buy for a melee character.