The plot centers around a light-speed engine that would be of immense value to one of the alien races given how fast it will allow them to travel through space. Their method of travel, however, is already drastically faster than light speed. Utilizing this engine would actually slow them down considerably.
This could have been made clearer in the movie, but the FTL travel used by the Kree is a network of "jump points", essentially stable wormholes. This is the same method used in both Guardians films. The jump points exist in fixed locations and lead to other fixed locations, and their distribution is irregular. The lightspeed engine described in the film would presumably permit travel at lightspeed (or faster) directly rather than by means of jump points. This would enable trips to places that might not be readily reachable by jumps, and therefore would make a ship with such an engine much harder to track and follow for someone without such an engine.
