I decided I wanted to try out a pure offensive team this season rather than try to build with a certain lead in mind, see what difference it would make. Unlike VGC formats, you have time to change your Pokémon and adjust during a match before the battle truly begins.
Like I said earlier, I already knew going in that I wanted to build Mewtwo for this season. The only reason I wanted to play Mewtwo specifically was that I accidentally planned around building Mewtwo in an earlier season not realizing it would not be legal that season. Mewtwo would be weak to Bug, Ghost, and Dark, so I would need Pokémon to cover those gaps.
Marshadow almost completely negates the rare Bug-type threats to Mewtwo and is capable of stealing stat changes from other Pokémon, giving Marshadow additional utility even if a match-up isn't one-sided in your favor. It's still weak to Ghost-types, though, and it's netural to Dark-type.
Finally, Magearna would provide coverage for that Dark-type that hadn't been covered yet, and turns the otherwise Ghost pure weakness into a neutral option for the team. Magearna also helps cover the weaknesses that Marshadow brings to the table