It brings A LOT to the game and by rights it should have been in game from the very begining. So with retinues:
1. This is a part of your army that you can customize to your liking.
Your usual levies are a mix of whatever troops can be levied from some holding. it will be something like 50% LI, 30% HI, 5% LC, 10% archers... It is quite hard to harness the military traits of the leader(since your three flanks will always be an irish stew of different troops)
2. This is a part of your army that is sworn to you and only you.
Whatever your liege does with your levies - these men will always be under your command, so you may desire to actively help our liege to win the war or to conuire your neiboughrs lands while his levies are far away.
3. Retinues of your vassals makes them able to bring significant force to their rebellions. It makes game much more interesting.
4. you can whack rebels with your retinue, without straining you relationships with vassals or collecting those levies of 43 men each into one host.
There are two more mandatory additions to the retinue system. I hope they will be added in the next DLC:
1. Vassals should actively use their retinues in global wars: It makes no sense for duke to just sit on his retinue while foreign enemy is pressing claim on his very lands. AI should be improved to allow vassal retinues to actually help in liege wars, if vassal is so inclined. Just like in case of Crusades.
2. People should be able to lend their retinues as mercenaries. It was quite common in that age and it would give mercenaries some more realism. For instance as a Duke of Vestergotland I would love to gain some experience and cash by participating in Byzantium civil wars.
