A high voltage system (i.e an EV with batteries) with downstream capacitance can be exposed to damaging in-rush current when the system is first turned on. If this current is not limited and controlled, it can cause significant stress or damage to other components in the system. A precharge circuit is used to limit this inrush current to slowly charge the downstream capacitance.
Precharging increases the lifespan of electric components and the reliability of the system as a whole. A precharge circuit allows the current to flow in a controlled manner until the voltage level rises to very near the source voltage before the main contactors are permitted to close. In electric vehicles a precharge circuit is used every time the vehicle is turned on.
Precharge is described in a lot more detail in this Sensata Whitepaper (PDF).