You hace a pencil black and white drawing scanned and you want to add colour without destroying the pencil art. So you create a new layer over the background and start painting your colours there, maybe with layer in multiply mode.
Then, for some reason, you inadventedly change active layer to background. You keep painting and then realize half of your painting work has been done on the pencil art layer (the background) ruining your work!
The solution is locking the pixels of the pencil art, so, if you inadventedly try to paint on that layer, the cursor will become a forbidden sign and you will get a pop up warning if you try to paint.
The problem is apparently you can't lock the pixels on the beackground even if you have pixel locking (or all locking) activated. This doesn't seem to work in the background layer (maybe because it is not really a layer).
The workaround is create a new layer from the background so pixel locking works, but I think Pixel Locking should work also with the background without having to convert it into a layer. Some lockings like transparency locking have no sense in a background, but pixel locking makes perfect sense.