Sure but only on the front and only if desperate and only if you're prepared to probably buy a new wheel after your ride. I've done it on the back for short periods as well (with the same caveats) by lifting my rear end of the saddle.
As for why, well sometimes it is raining like heck, your patches don't hold, or you get multiple flats, or your cell phone isn't working and even if it is your honey isn't going to pick you up, etc. There are memorable rides were just getting home is all that matters.