User blog comment:Kajune/Why did Lucifer come back?/@comment-26992577-20160225145131

From the writers perspective they probably wanted to bring Lucifer back as a fan service. They also had virtually no storyline for Castiel, so they gave him the role of Lucifer.

From the in- universe perspective: Lucifer has no idea how to deal with Amara without his father, she could right now just walk into hell, kill Lucifer and take over hell. Lucifer just wanted to leave the cage but seems to have no plan how to deal with the Darkness. I think he either dies this season, finally teams up with Amara or will be the next season's big bad once they solved the issue with Amara, which would be a bit lame.

So far, he showed up more frequently than in season 5.