Roles to grant when membership starts?

There is a role settings in Membership plans to grant membership to assign a role when membership start? is this needed? because all paid members on our website are authentic users and there isn't a role to pick for this under the role settings?

Basically, on anonymous users can register and they will not have access until they pay. Once payment has been processed and completed, they should become an authentic user and will be able to log into the site.

How is this to be setup in the membership plans? is there any documentation?

Regards,