The bar itself has no impact on the upcoming match. It is a feature in the game that aims to inform you, the manаger of the club, as to what can be improved in regards to the upcoming match.
A quick example is the message: "Some set-piece takers aren't set in your squad"
Whenever my players get too old, for what I currently need - subjectively, I would sell them off on auction or just sack 'em. Well some of these players are set up to execute penalties, corners, etc. If I haven't set these before the match, the game will automatically decide who to execute these events, but it's obviously better if I myself decide this by allocating a man for each job. It's common sense. I might have noticed that Player H is better at corners than Player B so I'll put Player H as the corner kick guy.
Even if I leave the captain slot empty it will prompt the Prep bar to notify you that there are missing set-piece takers. (Before someone asks the question about Captains - It's there for realism. There's no statistical gain from having a Captain.). But this will slightly nudge the Prep bar down as it looks at the overall picture and not the details. But hey, that's why your Club has a Manager.