Depending on what slot your skills are in, they have other abilities.

Slot 1 skills are usually a type of evasive attack. Although their damage is low, they will move you from danger fast.
Slot 2 skills are small area of effect attacks that will give you super armor status while executed. In this state, you will be immune to becoming stunned when hit.
Slot 3 skills are unique skills for each character.
Slot 4 skills do massive amounts damage in wide area and break super armor. They can be used to interrupt a boss charging a big attack.