Watt = Volt * Amps
in this case Volts are always 5
a device draw amps out of the charger as much as it was build for or as much as the charger can provide
if the charger can provide more amps and the device can draw as much charging will be faster and the device is likely to heat up a bit more
if a charger can provide a lot of amps maybe 10Amps which would mean 50Watt
then the device you charge will only draw as much as it was build to making the 50watt charger no better then a 12Watt if that is as much as the device can draw out
Older Apple adapters include the USB to Dock connector sync cable. The newer 12 watt power adapters do not include a cable which makes them cheaper. Even the newer 5 watt iPhone/iPod chargers are cheaper since they do not include a cable.
Apple probably dropped the cable from the adapters to eliminate a little confusion with the release of the Lightning connector on newer products.