Match preperation

Once you have full bars in all areas of match preperation, do we still need to use it.? what gains comes from using this.?

Yes, you need to use it, but you can set it to 'Very Low', which is enough to maintain Match Prep if you're prepping for two tactics. Match Prep is basically the level of familiarity of your players with your tactics. The better they get used to it, the better they will play in-game.