They're really not at all visible to car drivers. If you want a patch of light on the pavement as a guide for cars, there are much better options out there (such as the Down Low Glow or Bike Brightz). They will create a pool of bright light at the ground.
How does creating a bike lane behind the bike improve safety?
The idea is that it outlines the three-foot berth legally required in most States (some require four feet), and hopefully makes drivers who are unaware of the laws regarding interaction with cyclists on the road think twice about passing dangerously closely.
I'll assume there are some folks here who don't know why passing closely to a cyclist can be dangerous, for the sake of being thorough.
There have been instances of drivers who judge 'not hitting' a cyclist by the width of the cab, ignoring the mirrors at the side of the vehicle, and ultimately striking the cyclist with the mirror. Some people judge the 'not hitting' distance by the width of the cyclist's torso, ignoring wider handlebars, panniers, etc., with the same result. In neither scenario could I necessarily say that hitting the cyclist was deliberate.
There's also the issue of the wake created by a much larger, faster-moving object. The air forced around the car can easily disrupt the cyclist, sucking the cyclist towards the vehicle or blasting the cyclist off the road. Three feet can make a huge difference.
Besides this, there are some unpredictable riders out there. Though it's very unwise to vary one's line while being passed, sometimes it does happen. That three feet can make a huge difference.
Also, in light of the amount of harassment cyclists receive (people screaming at them, throwing things at them, swerving their cars at them, and on and on and on), giving a bit of space is nothing short of just plain polite.