Creative uses of descriptors with no real game effect are freebies: no extra effort or victory points needed. Situations where creative uses of descriptors have a significant game effect can be handled as power stunts: pick the effect that best suits the desired outcome and treat it as an Alternate Effect of the power the hero wants to use, with descriptors assigned as appropriate. If an electrical-controlling hero wants to use his power like a living defibrillator to save a heart-attack victim, for example, that can be a Healing power stunt. The hero uses extra effort (and possibly a victory point) and gets a one-shot use of Healing to stabilize the dying victim.
Descriptors help to bring a collection of effects and modifiers to life, differentiating them from similar (or even identical) configurations and making them into distinct powers. Although descriptors don’t always have significant game effects, they’re perhaps the most important element in providing color and character to the powers of heroes and villains.
Descriptors do have some affect on game play. In particular, descriptors often govern how certain effects interact with each other, serving as convenient shorthand to help define an effect’s parameters. For example, Immunity and Nullify work against effects with specific descriptors; if they were limited solely to things like effect type, it would leave out a tremendous range of options, like “Immunity to Fire” or “Nullify Mutant Powers,” which are important to the source material.