I may not be 100% right and I hope a hair pro will correct me if I'm wrong, but I think a colorist uses this when a person's hair is light and if they want to go darker a filler will add pigment so the color won't lighten or fade too much. Say you have highlights and you want to go dark brown, they might add the filler first to "fill" the hair with pigment, then apply the darker color. This is actually what I may need done next time I get color. I went a nice shade of medium brown couple of months ago and it lasted a couple of weeks and my highlights were back. The color faded completely. Terrible.