Below is the text from salesforce help
Subtab apps are the various sets of tabs available on specific pages, such as users’ profile pages.
Is there any use case of having subtabs under user profile page?
You can’t see subtab apps anywhere else at the moment, as there is a fixed set of subtab apps all associated with profile pages:
I’d say the use case would be tabs that users occasionally need to access, but they don’t want to clutter up the main Salesforce UI with the tabs or another custom app. Administration or monitoring pages for example.
There’s more information in the customizing chatter profiles document: