When is phase needed in DFT?
From the perspective of filtering, a common LTI system will weight each components in the input signal: (1) weighting means changing the magnitude values (it is apparent in multiplication of two complex numbers), thus it is often to observe the magnitudes (this does not mean the phase is not important; (2) of course, phase information is also part of the whole signal, and although the phase information may be also changed, but the phase is not apparent; (3) we can consider a FIR filter, and compare the changes of both magnitude and phase.