A dependency diagram is used for light-weight project management and planning.
Coordinating lots of people is hard. This technique gives you a really visual way to represent how people will need to interact to achieve an objective.
Plans change. Since it's a light-weight diagram, it's really cheap to draw and more importantly, it's cheap to change.
Colors are used to indicate the status of completeness of each item in the dependency diagram. Grey is pending, orange is in progress and green is completed.
A dependency diagram helps you plan a complicated sequence of activities that depend on each other. This is usually helpful when there is more than one person involved in achieving an objective (which is often).
It's great to keep track of how you depend on tasks outside of your control.
This educational board explains what it is, how to create one using Miro and gives a real-world example in the form of the journey for the evolution of this dependency diagram.