Why does Japan have police? Should it have police?

If it should not, please explain the reasoning. It can't be the reasoning you gave for the U.S., so then also please explain how you know that the answer for Japan isn't also the answer for the U.S..

If Japan should have police, please explain why we cannot aspire to have the same thing they do. (Not saying it will be easy, but the "I have a dream" speech wasn't about what was easy; it's about what is right and what is worth striving for.)

(My requests aren't necessarily impossible, but they're very important as one of several sanity checks that should be performed before wholeheartedly adopting what appears to be a very radical proposal. Also, note that human societies appear to have had police-like functions for roughly as long as we've had large organized societies. E.g. https://www.ancient.eu/article/1104/police-in-ancient-egypt/)