How Important Is It to Have Monsters in Your Book?

There's a lot of inconsistency about how fifth edition monsters are being done over all. The big question i have is, when you're doing a book like path of the pram, plain breaker, how important is it to have monsters in there? How much can you tie ito existing monsters? I think it's probably useful, because this is such a weird place, you want to definitely have a handful of monsters. But  i almost wish monsters could be separated from mechanics, though you really can't.