Witnesses are not suspects but they do have some information about what they saw, what they know. They can use this to trade for information, freely share it, collaborate etc.
Who would use it?
Last minute guests that can not commit to being a suspect.
Younger players that may not be able to grasp a full suspect role.
The host who wants to be more involved but is busy planning a kick-ass party!
Hosting a party for one of your kids? Use an optional pack to drop extra hints when you drop in on the kids.