It's both a night light to help you find your way to the bathroom at night, and it's the light you see, when your car is properly preventatively maintained...and you can bask in the glow from the security it gives you. Kind of makes me cry what a warm feeling that is!
This, btw, is not to be confused with the Mango light, which spells doom and foreboding, especially if you ignore it. We've lost many e46s because of failure to see 'this' light...aka the low coolant warning light.
The PM light is a spiritual light--only believers can see it!