How does a Dyno Really Work?

I get what your saying dyno doesn't measure hp directly, only rpm and torque (which is power) which you have to calculate hp, but don't understand why that's a big deal, you know how many things can't be measured directly? But we still know about it and how it works, we calculate watts by reading volts and amps.
It's not a big deal to me. I'm just making the factual point that torque is the only thing they dyno takes a direct measurement on.