While I am all for Necrozma being either or, Lillie tells us that Necrozma suffers without it's light and encourages you to fuse it with Nebby. That sounds like Necrozma benefits from the fusion.
However, you could argue Solgaleo and Lunala benefit from the fusion via stat changes, therefore being potentially harder to Kill, and extra moves, most notably Swords Dance and Rock Polish Turning Solgaleo into an absolute monster, and the use of Light That Burns the Sky. :P