If a Power logically requires a skill to use it efficiently, the skill must be bought separately. For example, the superpower Flight allows a character to fly, and usually no skill roll is needed. But the ability to make intricate maneuvers in close combat without slamming into a wall requires a roll against a Flying skill. (The GM may ignore this and simply say that no roll is needed for any flying maneuver with a Flight Power.) Another common skill is Throwing: hurling balls of fire or bolts of energy at a foe. Or the GM might rule that being able to aim and accurately release such energy comes with the power for free: no roll needed, it automatically hits the target every time unless the target makes a Good Dodge roll (see Combat, Wounds & Healing).
This can be especially true with magic: the ability to cast spells at all may be a gift, but to do it right is a skill, or even many different skills.