[nemof]

The primary benefit is weight reduction:

http://www.reactionengines.co.uk/sabre_howworks.html

This approach enables SABRE-powered vehicles to save carrying over 250 tons of on-board oxidant on their way to orbit, and removes the necessity for massive throw-away first stages that are jettisoned once the oxidant they contain has been used up, allowing the development of the first fully re-usable space access vehicles such as SKYLON.