The PowerBlock contains a rechargeable Lithium Ion (Li-ion) battery. The battery provides about 3.7 volts. This is not enough for the BrainBlock. The PowerBank has a circuit that boosts the voltage up to 5V.
The circuit also protects the battery while it is charging and being used.
Below is a picture of an 18650 Li-ion battery next to a smaller AA battery. AA Batteries are only 1.5V compared to 3.7V of Li-ion batteries.