Welcome to FlyerTalk,
dietofly!

^
You may want to check out the Mileage Run forum, and especially the stickies there, which will have a lot of information. I think the best way to quickly demonstrate the usefulness is an example. I, for one, enjoy flying. For example I can usually book something like TPA-IAD-SFO-SEA-SFO-IAD-TPA for around $200, which nets north of 15K miles with elite bonuses. That means earning 120K miles costs me $1600. For 120K miles I can book a first class ticket on Singapore Airlines to Singapore and beyond, which usually costs tens of thousands of dollars.