The segments exist so that the airplane climb gradient can be enumerated for each configuration and speed schedule used (segment) during the climb to a safe altitude with the assumption of one engine having failed on the runway at Vef
Segments are used to build a profile that differentiates the performance at several steps, slats/flaps in take-off, gear retraction, flaps/slats retraction (level segment), then clean. The gross/net is used for obstacle clearance, which assumes all engine.
With OEI, the segments and profile are different, especially the level segment. Obstacle clearance is not provided for OEI unless specifically stated so.