I am currently reviewing the Nyquist criterion which says I need to sample the signal at 2 times the maximum frequency in order to avoid aliasing in the reconstructed signal.
My question is that in all the applications I've done, I rarely ever remember having to reconstruct the signal after sampling it.
For example, in controlling a digital controller, all I have to do is to sample the signal and send the sampled signal to the controller. End of story, no reconstruction required.
In other applications, I sample the signal to analyze its frequency content. After looking at the frequency distribution, the signal is completely discarded. No reconstruction required.
Can someone make it clear why do we need to respect the Nyquist criterion if no reconstruction is needed (so no worry about aliasing effect)?