Just to agree with the other answers, clone recipes allow you to start with a recipe you know you'll like, and also give you an understanding of how the grains, hops, yeast, etc. work together in the finished product. The Two Hearted clone you can find on here is the base of my grain bill for all of my IPAs and APAs. I did tweak it a bit for my own liking, but it is still eerily similar. My favorite hopping schedule is a far cry from the original recipe, but initially brewing that clone stepped up the quality of my IPAs immensely.
Also, what do you do about a clone of a recipe you don't have access to? Getting anything from DFH is a special occasion for me, so instead of trying to find someone to ship me a case I'd rather just clone it. Same goes for RR, Surly, etc.
And of course homebrew is usually cheaper than buying commercial beer. And I'm a cheapass that loves brewing.