I agree with Headinclouds. OW award increases the choice of carriers. Quite often AA has no seats for JFK/BOS-LHR but there is an abundance of seats for the same route on EI with a stopover at Shannon.

One of my latest trips was ticketed as BOS-SNN-LHR-HEL-LHR-CDG-BOS. A straight AA flight to LHR was not possible on my chosen dates due to no available award seats.