Need? No. But it's convenient, especially for scenes that can take a little more time to access - like when their lust gets high and they come to you for relief. It's repeatable, but it just takes time to get there. Oh, I want to watch this scene with Kali but she's at school so I gotta click around and advance time. Even for the scenes that don't require build up, it can be inconvenient to navigate through the game world which is designed to be immersive, rather than navigate through a menu which is designed to be convenient.It doesn't include repeatable events. You don't need a gallery for that.
Maybe Harem Hotel just has too much content and a gallery would be way too much work? Totally understand that. It's a great game without a complete gallery. If it's not too much, though, it would be wonderful to have.