They molt for the same reasons your hair grows out. Feathers don't last forever. They get worn, broken, torn out, and need to be replaced. That's what the molt does. The old feathers fall out and new ones grow in. Several times as they are growing up then usually once a year afterwards.
When they molt can vary, but the better birds will do it late and fast. Typically from late October through December. Some start molting in late summer though. Unless you're using lights all but the best producing birds won't usually start laying again until the days begin to lengthen late winter to early spring the next year. This is why for birds kept for practical purposes you want the ones who will wait until late fall or winter to molt.