When you are setting up users in your account - you would have requirements to allow logged-in users/ verified customers to be able to view or submit tickets. In some other instances, you would want them to easily access the content of the ticket using a public ticket URL that has no signing in hassles. 

For all these configurations, please navigate to Admin -> Channels -> Portals -> click on the Settings tab to see all the user permissions for tickets, solutions, and forums.