Why you need to use the spacer. Hopefully this makes sense. When the brake lever is at rest, it holds the brake light switch in. When you pull the brake, the switch pushes out and the brake light comes on. When you have the assembly apart the switch actuator is out. If you don't have the spacer or at least pull the handle back when reassembling, the lever will break the actuator off and you will have constant brake lights.
I'm not at my bike, so I can't confirm but I believe if you use a flash light and look between the lever and the throttle body while applying and releasing the brake lever you will be able to see how the switch works. As stated previously, the switch actuator is very easy to break, speaking from experience here, so be careful.
:USA