Use it when investigating a problem, to identify and select key problem causes to investigate or address.
Use it when the primary symptom (or effect) of a problem is known, but possible causes are not all clear.
Use it when working in a group, to gain a common understanding of problem causes and their relationship.
Use it to find other causal relationships, such as potential risks or causes of desired effects.
Use it in preference to a Relations Diagram where there is one problem and causes are mostly hierarchical (this will be most cases).