Why Gray Bars and Sometime Black Bars?
Plain and simple the gray bars are used by the TV manufacturers to reduce the phosphor aging difference between the SD picture and the pillar box areas of the screen. If the pillar box areas of the screen were left black, the phosphors would not age as fast in those areas as the picture area would. Using the gray pillar box areas will cause the phosphors to age roughly the same as the picture area.
Well that's a whole lot more technical (and longer-winded) than
gray is better because there actually is a signal and therefore less chance of burn-in.
Not to toot my own horn, but I am reminded of Mark Twain's famous line on brevity as a (time consuming) virtue- It goes something like this "Sorry this letter is so lengthy, but I didn't have time to write a shorter one". Maybe sometimes knowing the concept w/o the details is an advantage ;~)