I've wondered that myself, but I think the idea is, if they set a low starting bid with a reserve, they can see how much people are willing to bid up to, so they could consider lowering the price if and when they relist the item. If they start it at their reserve price and get no bids, they wouldn't know if noone was interested or if the price was just too high.
Pure speculation of course, I personally wouldn't use a reserve in that manner because eBay charges a fee for reserve auctions but you can set the minimum bid to whatever you want without any additional fee.