I just think of the prisoners as moving florins. Prisoners in medieval warfare were taken for the ransom, this means higher ranking knights, etc. would be more carefully taken care of as they were worth more alive than dead. You might think taking prisoners instead of killing them all was a moral accomplishment in those days, but actually it is a financial/economic
decision.
Low ranking prisoners were the farmers/peasants who made up the bulk of the armies, and the winners need them to farm the lands they take. High ranking prisoners gave their word not to attack (for a period of time) and they kept it. After they were ransomed they might come back after a year and be captured again.

mfberg