One purpose, for the Visitor point of view, is to show a property 'who are in this photo' And show names separated from the rest of the tags. Also search in tags could have search on names, search on places, etc.
Places could be hierarchy, like location inside city inside country, so if someone search for a city group it can include all locations from that city automaticaly.
For the admins could have more flexibility to organize tags.
There are more features i can talk about when using groups of tags.

that's taxonomy problem/philosophy: Piwigo only use 'Album' as item to receive contents and to be ordered