I will guess and say this is driving a DC series wound or permanent magnet motor, where the commutator will provide the voltage pulses to commutate the SCR, and the capacitor bank is small enough that the ripple voltage across it is very large, so that it basically will deliver full power into the load till the motor is running, then the ripple across the capacitor allows the voltage across the SCR anode to drop below the cathode ( held up by the motor back EMF) so it turns off for the rest of the cycle, then it will turn on again on the next cycle ( happens at twice the mains frequency because of the full wave bridge) to add power to the motor.
Speed control will provide a basic rough constant speed providing there is enough load on the motor, but it will hunt at no load as the motor speeds up. Driving a fan or a machine spindle is the likely applications, where you want speed control but precision is not too critical, and there is always something absorbing energy.