I spray the inside of mine with that "hammered" black rattle can stuff from hardware stores. It adds texture to the inside and makes anything I glue inside stick better. I also llike the look of the black inside.
I orginally thought it would help with any glare. Same reason we rub black polish under the eyes in football. Not sure there is any benefit in that area though.
Then, like Tray, I just think it looks better when people look inside the lid.