It will make room for a bigger battery and screen
It may not seem like your headphone plug takes up that much space relative to the size of your entire phone. Nonetheless, it occupies precious real estate, especially when there's pressure to pack in a crap-load of advanced technology into an increasingly low-profile package.
Without a headphone jack, Apple can use that added space to squeeze in some other in-demand elements. Wish your battery could hold a charge for longer than half a day? This could potentially free up enough room for a slightly bigger, longer-lasting battery. Getting rid of it also opens up the possibility of a screen that stretches from end to end, which makes those rumors that Apple's ditching the home button in the next next iPhone a bit more believable. It's even a slight blessing for your pockets, since it allows for a thinner phone.