i supinate pretty badly. without cants my knee wants to drive to the outside of my ankle unless my stance is less than 12". it isn't comfortable, and it probably wouldn't be good for my long term joint health.
to go back to the person who said if your bindings were inline, you wouldn't need cants... that emphasizes this point. if that were true, there would be no skiers or mono-skiers who use cants. i sure did when i did both. i was never without cants in my ski boots.
cants allow you line up you knee so that when you drive it forward its over your ankle. that gives you a more efficient application of the gross power of the quads, while also allowing you to engage the finer tuning your ankle can provide.
can you ride without doing that? sure. but i submit that doing so will fatigue you more quickly, and potentially produce additional joint wear, needlessly.
some people are lucky enough that their knees line up quite nicely at 19 - 21" and the angle their board/boot sizes dictate. but many don't... many pronate, supinate or have valgus deflections that make the knee not quite align over the ankle. of those, many can compensate simply by working harder... but just because one can do something doesn't mean its being done in the most efficient manner.
because the most efficient stance width is partly a function of the board itself - canting allows one to ride the board close to its optimum configuration while also approaching their own physically efficient position.