Why do you choose the current based equation vs. the voltage based equation to calculate the max power?
In that case, because we had current to work with. We knew the limit of the fuse (2A), we knew the resistance (2 ohms) and we wanted to know the power we could deliver to the load with that current.
We can flip it over and calculate the theoretical maximum power that your 12V supply can deliver with the other equation- 72W. Okay, great, that'll certainly get things hot enough, but does it exceed the current limit of the supply? Easily figured out if we use the P = I*V equation. 72W = 12V * I => I = 6A. Hmmm, not going to fly with a 2A fuse and, although it might work briefly with a 2A limit transformer, it will probably end in tears eventually.