Firstly "u" is not a word, please be aware that our forums require the use of real words for the most part.
The backlight while plugged in option is very useful for any player that has a dock, is used with speakers or a home system, or otherwise is played while plugged in as well as portable. While plugged in, it's often nice to have the screen constantly on so that you can read it without having to press buttons on your player. While on batteries, clearly you might want it different to save battery life.
Just because it's useless to you doesn't mean it's useless.