Low contrast - you expand the contrast by either just developing longer, or a combination of less exposure and longer developing time.
High contrast - you contract the contrast range by over-exposing (to capture shadows), and less developing time, possibly with longer agitation intervals.
That is the way to achieve normal contrast from all kinds of lighting conditions, and is one use of expansion and contraction.
The other use is that you wish to achieve something different than normal negatives. You could want more contrast even from a normally lit scene, in which case you could either develop more, or combine lower exposure with more developing time, which stretches the contrast range out far enough that you exceed the limits of your printing paper/developer combination, for some pure paper white highlights, and some pure black low values - both without detail. In some instances this could be desirable and preferred to a normal contrast negative.
Now we've moved away from the realm of technically correct, and into an area of visual preference. Both are equally valid in my opinion, and a question of what we like or don't like. It's so important to wander into the unknown sometimes, and try to push our materials to the limit and beyond, so that we can find out for ourselves what we like and don't.