Difference between sharing to individual/groups and on role level

Difference between sharing to individual/groups and on role level

I am a bit confused as to what the difference is between sharing forms and reports in an application to individual users or groups compared to giving access to users based on the roles set in Users & Profiles.

For my specific applications, under the Share menu, I have set the entire application unshared. But under Users & Permissions I have instead given permissions for all users to access which in return allows all my users to access the forms and reports. I tend to believe, that administrating on a role level, as much easier than controlling individual user access for every form and report.

So question is - when is is needed to BOTH share the applications AND grant access on a role level?