Learning by doing is a great concept, yet certainly it is not by doing only that we learn.
The idea of experiential learning originates from David Kolb, and it dates back to 1984. Kolb suggests a learning cycle that starts with action (the by doing part) and ends with action (a new one, hopefully), but also includes other fundamental parts.
Without appropriate space for reflection (what has happened?), conceptualization (what does what happened mean?) and experiment (what are we going to do now?), we are simply stuck in a reality of actions without learning, and probably also without purpose and direction.
From that cycle, I believe it is possible to expand to clarify the role of leadership in learning.
On one side, the leader kicks off the experience with the formal or informal act of delegating. On the other one, it facilitates the interpretation of “what happened”, particularly for what concerns reflection and conceptualization.