Simple explained.

A brushless system reads the position of the rotor so the control (ESC) can optimise itself not to outperform the reaction speed of the rotor.

With sensorless it is done by reading the generated voltage in the coils of the motor when a magnetic pole floats along a coil like any dynamo works. The amplitude of the generated voltage is determining the speed the magnetic pole floats by. At zero RPM the esc will not read a thing as also with very low rotations. At a certain RPM level the ESC can read the feedback pulses and will work optimised with the motor, below that the ESC is a bit guessing resulting in what we call cogging.

Sensored is using electronic sensors to detect the position of the rotor. These sensors work all the time over the whole rpm range giving a much smoother control, even from when the car is standing still.

A racer wants to have the perfect start and control to get the best performance. A basher does not mind using sensorless because he wants to go full throttle and does not even notice the tiny struggling to start up.