I guess that question is more about USB hubs than just plain connectors which are all same for given standard.
Explanation is easy, Powered HUBs receive power from external 5v adapter and not from USB port HUB is connected to. USB ports have limited power output depending on their generation and each port shares data bandwidth and power with all devices connected to it. If your sum of devices or one device in itself exceed the power of a single port, you have to give it power from another source to compensate for it.
Another example is using USB adapter to connect HDDs, some, 2.5" drives may require more power than single USB port can provide so have to be connected two two USB ports at same time (the other connector is only for power, 5v) or to external power source.
Other example would be connecting a 3.5" HDD which also require 12vDC as well as 5v, USB doesn't have 12v source so yet again external power which has both voltages is required.
Than there's USB2 limitation of cable length to about 5 meters after which it starts loosing power due to resistance and low direct current voltage. In that case more power has to be added and over 10 meters data leads have to be amplified.