A relay basically would turn your power window switch into a remote toggle switch with the main power coming from a heartier 12V source. Right now, 12V coming in through that pink wire is routed through the entire switching network to power the motor. That equates to more resistance, thus more power loss by the time it gets to the motor itself. The relay setup just allows you to turn on or off the relay itself to control that steadier and hopefully more robust 12V signal to operate the window motor. Net result- more power and quicker window operation. That's the idea behind it.