In essence banner and musician help you win in static resolution during combat. They are two main ways to win combats : wounds (actual killing you do in combat & challenge) and static resolution (banner, extra ranks, infantry outnumbering the ennemy, flanking, plus miscellaneous other bonuses).
So it all depends on what you want to do with these guys in your strategy. Personnally I don’t always auto include a command group for each units, sometime I just run a champion especially for shooting units, a champion and musician for light cav, full command for bigger units. Banners can be captured and award extra VP for your adversary.
For unit composition in our army, you can check out the “Let’s get competitive TOW thread” it contains feedbacks and battle reports. Start at post 720 for post errata discussion.