Why Large filter capacitors in a high-voltage supply look like a dead short the instant power is applied, so the inrush current can be tens of amps and can blow fuses, damage rectifier diodes, or weld relay contacts. A step-start circuit places a resistor in series with the transformer primary for a fraction of a second, limiting that surge while the capacitors charge, then a timer or relay shorts the resistor out for normal operation. The goal is a gradual charge rather than an instantaneous current spike.
Watch out Contact arcing is a side effect of the inrush current, not the reason for the circuit; the step-start is protecting the diodes, fuses and capacitors from the surge itself. Compensating for line voltage variation is the job of a regulator or a tapped transformer, not a step-start.
Step-start = soft start: a resistor briefly tames the capacitor inrush surge, then gets shorted out.