Modern site 'people' web part not very useful
The 'People' web part does not auto-populate with the group's members so it requires manual maintenance and therefore it's utility is limited as group membership will change and so this page information will be inaccurate.
Or maybe I don't understand its purpose...
Its as useless as you thought and I'm not sure why it's included either
We would like to show department group people on page, however it’s not supporting.
We are looking forward to having this functionality in coming releases.
Benoît Fournier commented
Make it dynamic based on group membership or on profile information like stated here https://sharepoint.uservoice.com/forums/329214-sites-and-collaboration/suggestions/31115959-make-people-webpart-filterable-refineable-for-retr
Right. There should be option to be able to configure this web part for dynamic values i.e. filter by Department etc.