BJTs are current based transistors. This means you must supply a small current to the base to allow a higher current (100 times as much) since our beta is 100A per Amp. This means if we wanted to switch a load of say 20mA we need only 0.020 A (0.020A is 20mA) / 100 = 0.00020 A or 200uA. So we only need a very small amount of current, 200uA to switch a load of 20mA on.
The take-home here is that the BJT allows you to switch loads on and off while only needing a small supply of current to do so, making them especially useful for turning high loads like a motor on and off with say, 3 AA batteries and a resistor.
