How did the lives of knights and nobles demonstrate the importance of warfare in the middle ages

In the middle ages Knights were those celebrated and selected warriors whose sole purpose in life was to fight in battles for their lords or train others to fight in that way.
Lords were those who gave these knights lands in return for their military services while knights protected and fought for them, there were different rules in different places, for example in some areas some knights had more than one lords while in others a lord may hold many knight.
It was always a struggle between lords to keep and draw those knights who were braver and better than others.