I don`t think it`s necessary at all if you have a good coating. I have about 20 vehicles at work that dont even receive more than occasional soap/rinse tunnel wash much less a maintenance product.
Dating back to January 2017, the ones with a good coating always look better than the non-coated vehicles (or the ones with lesser coatings), still clean and shiny (relatively). Tar globs flick off easily, greasy palm prints, light chemical stains, nature debris...all clean off easily in the occasional touchless tunnel wash. They dont necessarily bead like they used to or the surface behavior is otherwise diminished but end of the day, they always look better than non-coated vehicles.
I probably top mine on occasion for a few reasons, and it`s mostly kinda like why I like a foam cannon...whether it really helps is up for debate but it`s easy, it`s quick, it entertains me and it can`t hurt.
I also do it on occasion for a change of appearance I think. I used to think it was possibly all in my mind but after using PA Cosmic Spritz on wifes Blue metallic car last wash (after only using Overcoat on it since 2016) she came home that night and asked what I did different so it must have been noticeable.
And in the case of Cosmic, it`s just so enjoyable to use that sometimes it seems like I`m washing the car not so much to get it clean overall as to get the surface ready to use the Cosmic...its just fun for me, the purple color, the smell, the feel of it going on and buffing off...its just fun (and not too much effort

).
Who knows though...maybe living in a house with products on shelves, in cabinets and everywhere else, her mind is becoming as confusingly clouded as mine.