Not all knights came from rich and affluent families. It was one thing to possess armor and weapons passed down from one's ancestry, but most of the time, that was about everything they had. It was the same for Lord Bennett when he was younger. If he was captured on the battlefield, no one from his family could afford the ransom.
Knights often served their duchies for two purposes: one was to improve their skills, and the other was to gain some political power within the group they joined. The former was more for knights who came from families that weren't as rich and powerful, while the latter was for knights who came from families that were already stable in terms of their finances.