The usual reason is to correct sun-bleaching. It's better to use sunscreen though.
Some guys with very dark hair buy units with blond hair at the hairline and then dye it dark, to get the benefit of light-coloured invisible knots (it is hard to bleach very the knots on dark hair to the point of invisibility without damaging the hair).
Pin the system to a polystyrene head, and hang the head upside-down over the bath. Then paint the dye on with a brush, obviously avoiding the knots and the base. The excess will run down into the bath, and not onto the knots.
As an added precaution you can rub the inside of the base with a generous helping of Vaseline. It will go through the lace mesh and insulate the knots from dye.