When you want to build a shared, holistic understanding of a multi-dimensional problem (especially to bring together perspectives from different stakeholders).
When you want to identify unexpected connections and gaps in your understanding of a problem system.
When you want to make smart choices about how to intervene effectively in a system for maximum impact.
When you want to mitigate the risks of unintended consequences of an action.
When you want to reduce the risk of developing superficial solutions – ones that may act as a temporary, short-term fix but fail to solve problems.
When you have designed a solution, but it is working in unexpected or counterintuitive ways, and you want to understand it better.