Mostly we use power transistors in robotics because it is to drive high power circuits like motors, relays. Let's compare normal transistor BC548 and power transistor D880 (datasheet).

You see that hfe is low in power transistor. In the above transistor circuits we have seen that we normally connect motor or relay in between collector and base. You see the max. Ic of the transistors, D880 = 3A and BC548=.1A Normal DC motor have current rating of 250 mA. So if I connect the this motor to the collector of BC548 then it will not able to drive the motor because max. Ic of BC548 is 100mA. But a D880 can drive this motor. This is the main reason we use power transistor. Second is the probability of damage because of the heat sink of the power transistor. If some short circuit occurs the heat sink will get heated, it won't get damaged. See the maximum voltage ratings of the power transistor. So if you are using some other transistor, see it's datasheet first to see that it will be able to be used in that circuit. The main thing you have to note is max. Ic and hfe(VCEO,VCBO see this also to choose Vcc).