For starters, what is motor soft starting? There's no industry standard. Many think of a "soft starter" as an electronic solid-state device for reducing motor voltage (and current draw) during startup. But there's no difference in principle between that and any other "reduced-voltage starting" method autotransformer, series reactor, and so forth. Each reduces the current (and therefore the voltage drop) in the power supply system during motor acceleration.
A second benefit, though unimportant in some drives, can be the reduced mechanical shock to equipment resulting from lower accelerating torque. Starter suppliers sometimes say that a third benefit is reduced energy cost. Reduced starting current supposedly means the motor "consumes less power." This is untrue. Accelerating a load consisting only of pure inertia requires the expenditure of a fixed amount of energy within the motor. At reduced voltage and current, the acceleration takes longer but the energy expended is the same.Whatever the type of starter used, two benefits result. One is the mitigation of a severe voltage dip on the system when the motor starts. System limitations or utility rules may require this.