This feature seems like it's almost there, as you can currently view purchases (even those made in store) from your MyBestBuy account, but it would be extremely beneficial to be able to reprint your own receipts and have it look like an actual receipt, versus right now, I have to print the part of the webpage that shows my order and hope that it passes for a receipt. Two places where this would really come in handy is with insurance (I keep receipts for inventory purposes) and I generally do a good job, but every now and then I misplace one.
Another example would be for issues that require repair under manufacturer's warranty and on some things with warrantis longer than a year receipts fade so one could simply go in and re-print their own receipt (Best Buy should also, on that note, extend how far back you can search receipts).
The most prevalent benefit of this idea, is that it reduces traffic at customer service and it's another way to drive loyalty (i.e. the MyBestBuy program) to customers that may not yet be members.