Use case roleplays highlight problems in your use case dialogues, so you are able to detect and correct them early. The audience of the roleplay can both see how the dialogue should work, and ask questions to ensure everyone understands the use case.
However:
Roleplaying is another checking practice that is subject to diminishing returns: pedants can make the whole process much more annoying and time-consuming that it needs to be, small errors in use cases are not that bad as they can be easily detected later, and many people object to the ritual humiliation of standing up and performing in front of the rest of the team. These kind of group activities can also be soured by managerial involvement, either by a culture of ``enforced fun'', or worse by turning internal consistency checks in to excuses for evaluating and firing staff members.