Soft question: Why do we need to reconstruct a signal? 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)?
The concept of reconstruction has nothing to do with the application, rather it has to do with the question: did I get the same signal that is really there. If you cannot recreate the signal back, that means the conversion process is loosing/modifying underlying information, which in most cases you do not want to happen.

So the confidence on the Fourier Transform, or to use the digitized data in the controller, comes from the fact that the loss of information is within the tolerance limits of your application. You can measure the quality of your digitized signal, by actually re-synthesizing the signal and comparing it to the original signal.