In numerous tutorials on the web they seem to be interchangeable. Is there any important difference I should know about?

EDIT: OK I found in tutorial i02 the following for the **[rfft~]**:

"forward real FFT. Like "fft~", but only one inlet (for the real part) and only the first half of the output signals are used. (The others are determined by symmetry: they're complex conjugates of the first half, in reverse order.) This takes 1/2 the CPU time of "fft"."

Can anyone explain the importance of this to me? I don't really understand what Miller is saying.