An example of how a knife wielding person is a deadly threat.
Importance of having distance. The officer did the right thing stepping back to get optimal distance, but officers are not obligated to retreat. He could've tripped on his ass earlier than he did, and could have been stabbed.
A "real life" example of how difficult it is to shoot at a moving target in an already stressful and dynamic situation.
A "real life" example of how one, or even multiple bullets, won't necessarily "stop" a suspect like you see in action movies.
The officer was very very lucky.