I thought I would come back with some constructive information rather than just my opinion.
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It's used for keeping track of all the important details in writing a book building worlds. It's probably more used for RPG campaign development.
That said there are number of systems out there that provide similar uses.
In truth you don't need it but looking at it will give you a good clue as to what stuff you should be paying attention to in your story to improve them.
Every story is base around some sort of conflict. Man vs Man, Man vs Beast, Man vs Nature/God, Man vs themselves ...
There is always something to over come. Longer stories have various smaller events of this nature.
Harem games get stuck in a rut if your primary adversary is the new babes X or some dink that wants a piece.
There are vastly better things that can be focused on.
Maybe, you are a conqueror or trying to free your home land...
Your harem could be your adventure party. You go around doing quests and dungeons and so on seeking fortune.
Maybe, you are the villain or slave trader ...
Doesn't matter what it is any part of those stories is vastly more interesting than having to deal with Joe Schmo.
To counter that the guy would need to be some major character and do something on the order of kidnap her from you.
To which you would then mount a rescue mission.
The point is in most cases it would take a fairly large character role to even make it worth having in most harem games.
You also need to realize there is a huge plot hole of some outside no body becoming any form of challenge to the MC.
The MC is a guy by design who can manage multiple relationships with women and they not want to break it off with him.
Then you are going to throw some piss ant into the fight and think he has any real chance. If the MC is the character the game makes him out to start with then the women should effectively laugh at the new character. The only way around that would be if the MC was the only real available male in the area to start with until Tom DiddlyDee shows the hell up.
Even then the chances of the women going for the complete outsider they know nothing about wouldn't be as likely.
You could always throw magic in as the excuse. The outsider uses magic. But then you have to come up with a way for the MC to counter it or do away with the outsider. The point is that becomes a mess.
The best answer when it comes to harem games is to stick to a larger main story and avoid the pitfalls of trying to bring other men in.