The Gainspan modules include capacitors on the power supplies. When power is turned off, the voltage on this capacitance is 0 volts. If power is instantly applied to the module, there will be a large inrush of current, trying to charge up the capacitance. This large inrush of current can cause a voltage dip in the source power supply, which probably isn't able to provide multiple amperes of current. Slowing the voltage rise or limiting the current with soft start circuitry protects against glitches on the input power supply. An example circuit is given in the Design Guidelines document.