There are definitely some benefits. If people are trying to sneak stuff past you if benefits you to not explain what you found or how you found it when you reject things. If someone submits an app containing shady things X, Y and Z and you just respond "no dice", now they have to either remove all of the shady things they were doing, or try to guess which one(s) were detected. They can proceed by trail and error, but in the process they will reveal a lot of their secrets because Google can compare the diffs of the submitted apps to learn about other shady things that the developers think might have triggered the rejection.

this seems like the non-cynical answer. thanks. don't know why i'm being downvoted for a reasonable question, but there you go.

It helps them to enforce rules arbitrarily depending of their current strategy and how important of a developer you are.

Just a WAG but I think it allows them to maintain total control over the playing field. They get to choose which apps best suit their interests in their market and apply the rules inconsistently to apps that somehow don't align with their interests.