As a reminder, these identify iPhones and iPads of individual users on the Internet and for ads as well, so are pretty easy for linking owner and location together. Furthermore, the hackers state that this is just a sample of the 12 million numbers that were stored on the FBI machine.
Not only that, but the Feds database there contained “user names, name of device, type of device, Apple Push Notification Service tokens, zipcodes, cellphone numbers, addresses, etc.”, every piece of info you are relaying to Apple when getting an iOS device, with the amount of details varying greatly by each person.
Earlier this year, Apple stopped the practice of iOS apps sending UDIDs to their servers, but IDs are all over the Internet used for various purposes anyway. Antisec says in the statemnet accompanying the leak that "We will probably see their [FBI] damage control teams going hard lobbying media with bullshits to discredit this. But well, whatever, at least we tried and eventually, looking at the massive number of devices concerned, someone should care about it.”