Knights became a distinct group under medieval European society. Why?

From the 9th century, there were frequent localized wars among different factions. Amateur soldiers were not sufficient. There was the need of good cavalry. Thereafter, a new section of people-the Knights grew. They were linked with the lords. The lord gave the knight a piece of land, i.e. fief. It extended between 1,000 and 2,000 acres or more. It also comprised a house for the knight and his family, a church and other establishments to house his dependents.

The knight paid his lord a regular fee and promised for him to fight in war. In this way, the knights became a distinct group in the medieval European society.