Well, he's their emotional support, so to speak, and also who they get their rewards from.
Motivating idols with sex related rewards...As expected of a Hentai Protagonist Idol Manager.
And he seems to really appreciate them too, if his thoughts during the trip are anything to go by.
I have to say though I'm quite interested in the scene where they 'came onto him', and that ending makes me want more of this trio.
Whether it's a case of "Art Imitating Life" (the names have been changed to protect the innocent{?}) ... in this instance, It'S GOOD TO BE THE KI ... er, PRODUCER (the lucky bastard).