If the fake time equals the current time, the frozen flag ("!") is ignored, see first conditional in gnupg_set_time.
One use case that seems sensible to me is to try to convince a long-running operation (e.g. a sequence of key generations) to all use a single timestamp. In this scenario, there's no interest in setting the clock to be some variant of the current time, just an interest in it remaining fixed across all the operations.