I can only imagine he has a price in mind (the reserve), but if he gets a few bidders who really want it, then he leaves the window open to getting more than expected doing it this way. If he did BIN then that would be the only price he gets, less a bit of haggling, when someone might have been prepared to pay more.
Also, I think in the old days you paid a fee on the start price, so many started their sale at 99p to avoid fees. Don’t think that’s the case now, but he might think it still is the case.
Actually, these days I don’t think you pay anything if it doesn’t sell (not sure if that applies if you have a reserve), so maybe he set the start price really low to get an idea of what price it actually reaches on ebay in a straight auction.