All of Human Form is for having Powers denied when you want them. The +2 version very specifically says "if you are only rarely able to take your powered form".
And it illustrates what it means by "rare" by saying certain
times of the day. Such as, say, at night.
And it's
not for having powers "denied." It's for having some powers available in one form that aren't in another--Will and Georgia have Human Form, and their shapeshifting is
entirely at will--there is nothing stopping them from turning into a wolf aside from, say, particularly tight clothing. Same with the Hexenwulf belts--an entirely at will change.