- Strength, Dexterity, Intelligence and Faith increase the power of heavy weapons, light weapons, sorceries and healing spells, respectively.
- Vigor, Attunement and Endurance influences maximal HP, MP and stamina.
- Vitality influences maximal equip load.
- Luck influences item drop rate.
There's some secondary uses as well (magical stats and luck influence elemental resistances, physical stats influence physical defenses, attunement gives you more spell slots) but those are the main uses.