+1 to those who recognize the value of this indicator (I'm currently testing to see if mine will work). Anyone who's ever been driving along a slushy highway/freeway and had a big truck cover you in brown slop knows what a dangerous situation this can be if you're out of fluid. I've had this happen with such thick muck that my windshield visibility has gone completely dark - like someone painted your windshield brown in 1 second. This is not a "dummy" light as some suggest (probably peeps from down low who have not had the thrill of this type experience!!).
And yes - of course I top up my fluid regularly so I should never get the warning but it's nice to know that it works in case I'm empty and don't know it (it's also tough to see the level given how the reservoir is buried in the engine compartment of a 12 focus.