That spring is for the auto light function. When this function is turned on(I think on that watch, you hold the light button down for about 10 secs until it beeps and you see some little icon on the display)it tells the watch that the watch is at an angle (as if you are looking at it) and turns on the light as if you were pushing the button. It is useful if your hands are full and you need to see the time in low light.
G-Shocks have this feature. If you take a G-Shock/Protrek/Pathfinder into a quiet room and shake it next to your ear you will hear a slight rattling. That is the spring.
If you never use this feature, you don't have to worry about missing it.