Imagine you're travelling around Europe with your wife and teenage niece, bringing with you the following devices, and that the hotel you're in only has TWO accessible outlets (even after you've unplugged the lamp and TV):
blackberry playbook tablet
2 android phones
2 cameras, each with their own proprietary battery charger
Now imagine that at least 4 of these devices can be charged in parallel from the same outlet (two via the USB ports, 2 or 3 more via the grounded 3-prong power ports). Imagine how happy your various fellow travellers - who depend on you for their tech support - will be to know their devices will have enough juice for the day ahead.
(Note that for high-drain devices like an ipad or Blackberry phone, the USB ports don't supply enough juice to charge. But a small AC-to-USB adapter allows you to use one of the 3 AC ports to charge via USB. Wherever possible, get an AC-to-USB adapter which has a high output rating, such as 1000mA, 1500mA or higher. 500mA is not enough.)