I know only one use case - it is needed in "Send Email" action when you want to use a standard Salesforce email template in it.

Usually in email templates users want to use merge fields based on some context record. WR brings that ability by providing a flow context record as a template context record (i.e. the same record used for a flow instance will be passed to an email template). The problem is that Salesforce has a very strange limitation: if you want to send an email from apex code using some Salesforce email template and you want to pass some record as context to that template - you must send an email to a Contact (standard Salesforce object). That's why we need to specify that helper Contact on the WR Settings page.

So technically it works as below:

1. In your organization you create a dummy Contact record with some email like "[email protected]".

2. Then you register that Contact on the WR Settings page.

3. Every time when WR sends an email using a Salesforce email template, that email is sent to a Contact specified on the Setting page (which allows us to pass a flow context record to a template). The real email addresses to whom you want to send the emails are provided in a separate apex method. I.e. if you want to send an email alert to "[email protected]" then in the end it will be sent to two addresses "[email protected];[email protected]".