In some of the skills, such as Perception, Disarm, Identify, etc. it doesn't make sense to go above 10 after you get to Grand Master. But other skills, especially Magic Skills, Bodybuilding, Meditation & Learning take into consideration how many skill points you have … for example, the Heal or Power Cure spell will give you a base amount of healing points plus more healed hit points for each skill point you have in Body Magic.Other spells, especially the Elemental Spells, Light & Dark Spells, also give more benefits or damage per skill point. The Day of the Gods Spells increases your party's Stats according to how many Skill Points you have in Light Magic.
The manual explains this concept pretty well starting on page 37.
