Why would I tick the Set user ID option in the Piwik PRO tag?
UserID may help you recognize visitors, even across multiple devices. Piwik PRO Tag Manager provides an automatic setup of UserID, you only need to create a variable that will be holding the userID value. The value can be anything you like but it’s important for it to be unique to avoid confusing visitors.
To see how to implement UserID step-by-step, take a look at our article that explains it step-by-step.