I work for a hotel company. It's not really excess inventory. We sell on Expedia, Travelocity, Orbitz, Booking.com, etc just like we do on our own site. It's all the same inventory. Now if it's getting close in and we have unsold inventory we might sell on Hotwire or Priceline. In the industry these are referred to as "opaque" since you don't know what hotel you are getting until it's booked. We would much prefer to sell on our own site and sometimes offer discounted rates since we don't have to eat the fees. The OTA sites force rate parity which prevents us from selling a room on Expedia cheaper than say Booking.com. This even gets down to the room type level so we can't sell 1 & 2 bedrooms on Expedia, but only 1 bedrooms on Booking.com. They have automated systems to enforce this and send out warnings. We can usually get away with selling discounted rooms on our own site though. I always laugh when I see a trivago add since for the most part they all will have the same rate anyway.
Expedia has pretty much taken over everything. They have HomeAway, VRBO, Hotels.com, Hotwire, Orbitz, Travelocity, trivago. I usually tell people to shop around and find something you like, then go to the hotel site and see if you can get the same or better rate.