Fuss I'm not sure what your question means. So let's go with the more charitable interpretation- you wonder why a crossover. So: Eric Norgarden of Magnepan told me the maggies do best when they are not tasked with making deep bass- that should be best left to a subwoofer.
A guy you probably hate has a video on that same subject which I found logically compelling.
So if the manufacturer and qualified others say get a sub and crossover I think I need to take them seriously. Having owned 2 pair of Maggies in my life I can tell you they struggle to make bass- at the peril of their mid speed.