Warranty? Customer Service? Reputable Dealer? Some people value that sort of thing.
On a side note, the reason I said Crutchfield is because they have the best customer service for online stores. For example, I bought a HU off of them.. didn't really like it.. I sent it back (they paid for shipping) and exchanged it for something else.. no questions asked. There was nothing wrong with it I just didn't like it. They still will exchange it for free. What other online store does that? Sure you pay more than most sites but what you are paying for is the all of the little perks they provide you with that goes with the sale.
Would I buy everything from crutchfield? No I wouldn't as sometimes I don't need all the speaker adapters and such for free and just need a cheap set of speakers. They are just a good site to add to your many sites of online stores because there are times when people want that sort of service on a product. Also, they have the most reviews and information about the products they sell. I always go to crutchfield to research a product im interested in .. not to always buy it there but to at least read all the info they have posted about it and check the reviews. It's a great site for information.