I could never figure this one out myself.  Tried to convince my mother and argument failed spectacularly.  There's really no advantage to spending bitcoin versus traditional fiat.  To me, anyway.  The value is in holding it, much like precious metals.  People will use it for dark markets and gambling, so that creates demand and a chance for price increases.  Hell, look at the price now compared to bitcoin's inception.  It's definitely taken off.