Northouse defined that leader has two category including assigned leaders and emergent leaders. (Northouse, 2019) An assigned leader is defined as a leader engaging with certain leading position such as CEO, principal, manage, or team head and everyone can recognize the leader by positions. Whereas Emergent leader is a real leader who really influences members of a group to move onto the goal regardless of having a certain position or not.
When I attended some class in the previous university, the professor suddenly divided the class into the small group of 4~5 people to let us try Marshmallow challenge, which compete to build the highest Marshmallow tower using some pasta and tape in 18 minutes. There were 4 members in my group and the other group members including two men and a woman were all from India. First, I thought they were better English speaker and one of the men said he was confident about the activity, so I would become followers. However, some time passed, two men started to argue to push their own way respectively. The other lady didn’t say anything and watched them. I tried to listen both opinions and it seemed that I had more geometric knowledge to design a tower. I tried to communicate nicely to point out their contradiction. I noticed both men were short temper and were not good at listening the other opinion. I emerged as a leader and leaded our discussion and activity and finally our team won the challenge with the highest record.
Northouse(2019) mentioned, “There are clear differences between management and leadership, the two constructs overlap.” Simonet and Tett (2012) who researched the overlap the difference between leadership and management found both leadership and management involve productivity, customer focus, professionalism, and goal setting, but they found leadership has some other peculiar aspects such as motivating intrinsically, creative thinking, strategic planning, tolerance of ambiguity, and being able to read people, whereas management involves rule orientation, short-term planning, motivating extrinsically, orderliness, safety concerns, and timeliness. I think leadership is more focus on followers as human beings who can improve and change by influence of leader. Manager is more focus how to use human resources to achieve the goal. For the prosper of the organization, I think strategic long-term planning is important.
I wonder what kind of situations do we need a manager? and what kind of situations do we need a leader? Leader can take over manager?
References
Northouse, P. G. (2019). Leadership: Theory and practice (7th ed.). Thousand Oaks, CA: Sage Publications.
Simonet, D.V., & Tett, R.P. (2012). Five perspectives on the leadership-management relationship: competency-based evaluation and integration. Journal of Leadership & Organization Studies, 20(2), 199-213

Thanks for your post on this topic. In response to your question, “I wonder what kind of situations do we need a manager? and what kind of situations do we need a leader? Leader can take over manager?” I think that in all situations you need a manager and a leader. For example, you need a manager to manage the people in order to achieve then vision set by the leader. You need the leader to set said vision and to motivate those involved.
Take our church model for example. My wife and I are co-chairs and then we have 4 directors under us: Pastoral Care, Program, Business Administration, and Children & Youth. While we need to manage these directors we also need to set the vision for the church. Just like our directors, while they need to mange their teams they also need to motivate them in order to achieve the vision set by us. Therefore, all of us are being managers and leaders dependent on what we are doing.