They make a fatter profit, and enable the publisher to sell the book twice, as many people will buy both editions.
Actually, quite a few publishers now publish straight into paperback, but it doesn't happen very often in the 'bestseller' market. Places like Mills & Boon only publish standard print editions in paperback because of the the number of new titles they produce - it keeps costs and overheads down and thus keeps readership levels up.
The latest thing is the trade paperback - essentially a larger format, paperback book, but with a heavier thickness and more glitzy cover than normal. Some say it's a way of charging hardback prices for a paperback.