By definition, a leader is one who has the ability and/or capacity to direct or guide. Ergo, it is necessary to have certain skills and information to help. Such skills and information can be acquired simply by watching someone excel in the given field and developed over time by learning from their trials and triumphs.
As a diligent follower of a good leader, you get first-hand knowledge and experience of the challenges that await you when you eventually are at the helm of a team. Also, it's been said that the best form of leadership is one grounded in service, therefore, a leader who has been a follower, would pretty much be in his element in this regard.
Furthermore, an admirable quality in a leader is empathy and one who has been a follower can easily empathize with the challenges his/her followers may encounter, as well as offer helpful tips to alleviate the problems, making the task or project reach completion efficiently and in ample time.
Conclusively, a leader, who is (or was) a diligent follower, has both an abundance of experience in leadership, skills and adequate knowledge and experience to empathize with his/her followers.