Simply shows how much of your packet data quota you've used so far this month.
If you have a pre-determined quota from your operator of how much packet data you can use in a month, it's a pain to first find the Logs application, open the packet data counter, then try and work out how much data you have left from the amount used so far.
Data Quota shows two simple bars: one showing how much of the data for this month you have used; and one showing how much of the month has elapsed. This lets you tell at a glance if you've been using data up too fast or two slow, so you can be more frugal or let rip!
On the first of each month (or whenever your quota resets on billing day), clear the packet data counter. For example: Tools -> Log -> Packet data -> Options -> Clear counters