The intention of adj. cam gears is to tailor the cams to your driving style/needs/wants.
As RPM changes so do the air flow dynamics of the engine. As RPM increases so does intake air velocity, or required velocity for complete cylinder filling. Things like overlap can have an impact on higher RPM power, more overlap can result in more power.
On a basically stock engine that is predominantly a DD, they aren't needed. But if you have them and get them tuned you can have a schizophrenic DD/track car.
Will they add power alone, no.
Can you gain power with them, some in certain RPM ranges.