What does the Thunderbolt Display offer besides a display?
The thunderbolt display actually has a number of additional peripherals in it:


Gigabit-Ethernet Network interface card.
Firewire 800 interface 
4 USB ports on a PCIe-USB adapter
Webcam ("Facetime") camera (connected internally via USB to the USB-PCIe adapter)
Speakers and a microphone (ditto)
It also has a laptop charger (magjack) for convenience.


This is accomplished via the fact that thunderbolt can carry PCIe signals in addition to the normal display image, so the peripherals in the display act like they're built into the laptop.


  Thunderbolt combines PCI Express (PCIe) and DisplayPort (DP) into a serial data interface that can be carried over a single cable. Thunderbolt controller chips multiplex data lanes from these two sources for transmission via a duplex Thunderbolt lane then de-multiplex them for consumption from receiver chips.




There is actually the potential to even have an entire GPU in the external display, though no current devices implement the option.