to set the mood and tone of the play and provide background information
to give some insight into the relationships among different characters
Explanation:
A prologue will work as an introduction. It will provide you general information for you to understand at least generally what is going on during the play when it begins. It can also provide some insights that will help you to understand better the behavior and relationships of characters and even how to feel in some scenes.
When needing advice about something. Like if someone watched their mom get hit by a friend. The one that watched their mom get hit should be able to talk to a friend about what to do next.
