What is the use case for key questions?

The main use case for keys and key questions is in Table (Subforms) for data grouping and export. Since these are multiple submission-type Forms, the keys are used to group the response set submissions together when using dual extraction (2 Users Needed to Process in your Level Settings).

With multiple submissions from two different users, DistillerSR will pair up the responses so a proper conflict check can be performed. If they don't have unique keys to identify the submissions, DistillerSR will compare all the response sets against each other - which will almost always guarantee a conflict. 

More information about keys can be found here.