For large organzations (we have ~45,000 users, not counting service accounts), it would be useful to be able to filter the results of a combo box or drop-down for selecting "Users" to only include specific groups (be they O365 groups or Azue AD security groups or groups from a SharePoint site).
The Person column type in SharePoint already support the last option there, but I don't know if the restriction would carry over when that field is surfaced in PowerApps. Still, even if it did, there is a legitimate need to filter by the other types of groups.
For example, we're a University with Faculty, Staff and Students. We have AAD groups for each of these populations (actually, two for Students: undergraduates and gradstudents) and there are many cases where we create lists or forms where users should only be able to select from one population (e.g.: only allow students to select from the Faculty group when submitting a request for a recommendation letter).