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What is it actually for?

It doesn't show any useful information except that it by default shows the application icon when minimized, which will confuse many new users.

How about using them for something actually useful like showing CPU/Network usage of the app?

Or what permissions it has to your system?

Maybe you can think of some better things. Post them down below.

Possibly could be user-configurable so one user could decide to have resource usage graphs, another could have thumbnails, another just the titles.
My suggestions:
*various window + process properties (window states, pdid, command)
*thumbnail
*icon
*title
*action buttons (raise/close/min/max, etc.)

ETA:
* app notifications/status entries (inspired by notifications in mobile platforms). Would require a new api for apps to expose items to be displayed. Some possibilities:
** job progress details for e.g. file operations or other things which show the progress bar in the task bar
** e-mail/IM previews for new/unread messages (possibly even with inline actions - delete/trash/open/inline reply)