System Generator: Does an fft block need a buffer?
It depends on your implementation whether you need to explicitly define a buffer first, but yes, FFT always requires some kind of buffer, since it operates on the samples out of order from the time those samples occurred. It can't just run on each individual sample as they come in.

It needs to either store a section of data for working on, or predict the future.  I don't know of any machines that can predict the future.