Purpose of assigning stores to users in SIM & how to purge if there is no corresponding user-role

In SIM 13.2.x, users need to be assigned to a store to gain access to the store but then additionally a role assignment for a user always includes the store number in the assignment. I am wondering what the purpose of the first step is (assigning store(s) to a user) since without the role assignment (which is also store specific) user has no rights within the store to perform any action.

As a related question, PurgeInvalidUserRoles.sh does not remove user stores where no role has  been granted to the user. This would seem to lead to giving a list of stores to a user that serves no purpose. How can these be purged so that the store access list for a user is in line with stores where they have been  granted a role and what users see in their dropdowns is a clean list?