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.
Does booking.com have any customer service worth the name?
I use them sometimes, but from a company with such aggressive / dishonest marketing I would generally not expect any customer service worth the name. IIRC their terms and conditions exclude all responsibility whatsoever if the reservation does not work out, It's always the hotel's responsibility.
Luckily I have never had a case where I would have really had to contact their support.
Yea, that is one of the value propositions of using a middleman/marketplace (conversely, why many of Google’s forays in this area are horrible). Most OTAs are majority CS staff.