From Fallen Sword Wiki
HP is one of the 5 attribute stats that determine the success or failure of combat.
The other attributes are Attack, Armor, Defense, and Damage.
HP determines the length of a battle. The more HP a player has, the more damage that player can take, and the longer they can stay in combat. However, while training and leveling up, HP isn't important much because we want to make sure from levels 1 to 185 (est.) that we kill the monsters in one hit.
With the Buff Wither, a player can half the opponent's HP at the beginning of the battle.