Retraining also has the benefit of conferring bonuses from where you retrain them while keeping the old bonuses - e.g. if you've got a unit of No Dachi with +2 Weapons you can retrain them in a province with a famous armoury and get No Dachi with +2 Weapons, +2 Armour.

It's not just a good way to replenish troops, it lets you upgrade older units.