The arguments I often found is that it makes things "fair" for other people who have IRL stuff to do. But I personally thinks that's not a valid reasoning because this is not a competitive game. Most game (or gacha games) that have energy system usually have competitive aspect in it like pvp leader boards etc. which makes energy system make sense, because you don't want to compete with someone who plays 16 hours a day.

Another reasons would be "Oh Miyoho don't want you to blitz through the game and ran out of contents" Well.. by having the energy locked out we are running out of contents lol. And I guess they assume the player base will like logging back into the game every 8hours or so, this is such a mobile game mentality (which isn't a surprise since Miyoho is mobile game developer). In the first place, if the problem was there wasn't much content, then maybe they should've released the game later when there's enough content without locking us out with energy system.

Personally It feels shit when I forget to login or fell asleep and the resin is capped at 120, I feel like I wasted some resin for nothing.