It's about being able to see whether or not your MacBook is fully charged without turning on the power or opening the lid, like when you shut down for the night and want to know that your Mac is fully charged in the morning. Without the light turning from red to green, you'd have no idea without turning on the computer.
It's about knowing that power is going into your Mac with the visual light, rather than needing the disruptive sound that can bother other people in the library or workplace, and may even indicate the Mac turning on when you want it to stay off.