Each of the six abilities is used in different circumstances.
Strength: Used for melee attacks and saves requiring physical power, like lifting gates, bending bars, etc.
Dexterity: Used for saves requiring poise, speed, and reflexes, like dodging, climbing, sneaking, balancing, etc.
Constitution: Used for saves to resist poison, sickness, cold, etc. The Constitution bonus is added to healing rolls. A PC’s number of item slots is always equal to their Constitution defense.
Intelligence: Used for saves requiring concentration and precision, such as wielding magic, resisting magical effects, recalling lore, crafting objects, tinkering with machinery, picking pockets, etc.
Wisdom: Used for ranged attacks and saves requiring perception and intuition, such as tracking, navigating, searching for secret doors, detecting illusions, etc.
Charisma: Used for saves to persuade, deceive, interrogate, intimidate, charm, provoke, etc. PCs may employ a number of henchmen equal to their Charisma bonus.
Designer’s Note: In a system that relies so heavily on the six abilities, it’s important for each of them to play an important role, to discourage dump stats. Non-magical characters tend to dump the mental abilities, for example, so I increased their usefulness.