Leadership is about positively influencing others.
In order to influence others, you need to have a good relationship with them.
One of the best ways to establish and strengthen relationships is for there to be trust between the parties.
Trust comes from reliability.
Ultimately, being reliable means that you do what you say you are going to do.
If you want to be an effective leader, you must deliver on your promises.
I don’t think it gets simpler than that!