Mainly the convenience of seeing many options in one place. Sure, if you know you want a specific Hilton in Seattle, then go directly to their site. When we looked for places to stay in Norway and Denmark last summer, booking.com was a great resource to learn about many options and the going rates. Once we narrowed things down to a couple options, we could look at the direct site or book through Booking since many smaller places bounced us back there anyway.