Why do this?
Making a Key Result into an Objective can allow for nested Objectives. This is referred to as a parent/child or Objective/sub-Objective structure. This is often utilized when a Key Result becomes too robust to function as is and needs to have its own Key Results. The nested Objective will now have its own Key Results and will contribute to a parent Objective.
How do I make an Objective from a Key Result?
You can make a Key Result into a Child-Objective post creation, by utilizing the "..." menu.