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 create an objective from a key result?
From the objective creation view, select the split icon to the right of the key result you'd like to convert.
The key result is now an objective.
You can make a key result into a child-objective post creation, by utilizing the "..." menu.