Opposites

"It is disease that makes health pleaseant, hunger that makes fullness good, and weariness that makes rest sweet." 

We learn from opposites.  You learn to understand 'up' from 'down.'  You learn the benefits of hot because of cold.  If one didn't exist, then the other would have less meaning.  You learn to appreciate and understand things better when you also experience the opposite. They can be points of reference to better understand yourself. To know an experience is to gain insight into your preferences, to be able to draw from years of testing, or living and learning.