If the student demonstrates immediate competence, then why not introduce the next lesson?
Because you have not briefed it!
North American teaching tends to be by osmosis, if you expose the student to things enough times, some of it sinks in. In Europe a more structured building block approach was introduced in 1917. You brief, you fly what you have briefed, then you land and debrief. Conducting unbriefed lessons is a waste of the students time and money.