If the logged-in user is not a default owner then he/she can only see his own cases. If the user wants to see other cases without being a default owner then they should be a case lead for those cases.
To assign a Case Lead Navigate to Cases > Select the Case
On the Client Detail tab click on the Edit button to make the screen editable
Expand the Client Management Details panel and click on the Case Lead Dropdown. It will only show the Active workers in the list.
After assigning as a Case Lead, log in with the user, and you will be able to see all the associated Cases under Cases Screen.