Basically, Retinues allow you way better controll over your army composition, and allow you to get a lot of heavy cavalry/archers.
Archers are very powerfull type of retinue, and retinues are pretty much the only way to have a lot of heavy cavalry.
You can try to make a one very strong archer flank, and defencive center+other flank.
That way, archers will very quickly overwhelm the flank and sweep the center and last flank.
You ca often deal literally huge damage, killing off 70+% of enemy army in one go.