How does a rectangular window affect the signal?
A rectangular window is typically used when you want to process a smaller slice out of a much larger vector of data, possibly zero-padding within a larger window (for better time locality, because a bigger FFT is too slow, doesn't fit in cache, or some other reason).

In the frequency domain, a rectangular window convolves a longer signal with a Sinc function, producing windowing artifacts that are sometimes called spectral "leakage" if the longer signal is not strictly/exactly integer periodic at the length of the window aperture.

A typical example might be that one wants to use an FFT of length 4096, zero-padded from a rectangular window of data of length 2048, in order to analyze a 43 millisecond chunk (one note? one phoneme?) out of a 48ksps audio recording that is an hour long.