Organizations are running more and more projects and programs in parallel. It is rare for these projects and programs to be completely isolated. It is much more likely that they are dependent on other projects and programs within the organization and will be impacted by changes to the budget, schedule, scope and/or risk of those projects and programs.
It is more important than ever to be able to understand these dependencies and to be able to analyse the impact of changes or what if scenarios. The dependency matrix helps you to:
Here is more information on Requirements traceability matrix, i.e. capturing dependencies between requirements.

The following screenshot shows a simple dependency matrix for a specific program:

This program contains a number of projects and a sub-program with a single project (QWS), represented by the columns and rows in the matrix.
The intersection of a row and a column represents a dependency. For example the intersection at the first row (ADF v0.1) and the second column (ADF v0.2) shows that project ADF v0.2 is dependent on project ADF v0.1 with a total impact of 11%. The traffic light next to the number indicates that: