Why did lords need serfs?

Serfs who occupied a plot of land were required to work for the lord of the manor who owned that land. In return they were entitled to protection, justice, and the right to cultivate certain fields within the manor to maintain their own subsistence.

Lords needs serfs to work for them.

Explanation:

Serfs are regular town people who raise crops and work on the Lord's land. The Lord allowed the Serf to raise crops to give to the Lord and the a small portion to the Serd. Sort of unfair but hey they got food so it was a win win. So all in all the Lord needs the Serfs to feed him.