In general, people don't bother to crush cans. Meh. I understand it. They're (generally) either too lazy, don't care, or aren't sure how or why to crush a can.
As to the "why", a crushed can takes up less space than a non-crushed can. This means that more crushed cans can be stored in a space than non-crushed cans.
As to "how", there are several methods. One of the easiest and least thought-required is to equip oneself with a boot or very sturdy shoe and squash the can. In an apartment building, this can be problematic. It can annoy neighbors, especially if done at "odd hours".
There is a work-around for this. If a can is taken in hand and pinched, not hard, but just enough in the center, and then rotated 90 degrees and pinched again halfway between the first pinch and the top and bottom, those creases make it easy to take the can in-hand and press the top and bottom so that the can folds in on itself. This type of crushing is not hard, in the least bit. It also tends to not be noticed by neighbors, which is a positive thing in an apartment block.
