I can filter by ratings and locations above a certain level for places I'm not familiar with, filter by refund policy, and then read reviews and see if there are any deal breakers. I don't have to fight with an individual hotel's booking page, I know TripIt will read a Booking.com booking email perfectly, and Booking.com now has all my payment details saved.

Edit: it occurs to me that this is much like the advantage that Amazon once had, for the period of my life when I bought pretty much everything via Amazon. Shame they've lost my confidence.