Why do many rheobas circuits include a transistor and not a potentiometer?

Question

Cooling systems

I continue to understand the basics of electricity. Question in continuation of the topic with a cooler from a 12 volt adapter ( https://toster.ru/q/329715)
Briefly. I power the cooler from the case from an adapter of 12 volts. I decided to look at options for implementing rotation speed control based on voltage changes (I didn’t bother with PWM). Almost all circuits use a transistor to change the voltage.
1) Why not just use a potentiometer? It just changes the voltage in the electrical circuit.
2) Often in the circuits there is some kind of resistor, which is designed to set the minimum voltage in the circuit. For example, here ( pikabu.ru/story/tishina_v_pk_ili_reobas_svoimi_ruk...Here it is the fourth in the section on details. What is the principle? .. In theory, the resistor simply reduces the current. Or no?..

Answer 1

Calculate the power dissipated by the potentiometer.