At one point, whites were the only recognized variety, and blacks came next. Whites had the best type for a long, long time, and at that time, to improve type, you needed to use whites. As the coloured birds' type became better and better, breeding back to white became not only less needed, but less desirable, as you would not know what all genes a specific white would bring into the mix.
If you KNOW what genes the white carries, that can change the story. For example, if you hatch a white from a blue/splash pen, you know htat the bird will carry at least one copy of blue. Likewise, if a white came from a grey pen, you know that it is silver gened, etc. But any random white from a white pen, or purchased, you don't know, without experimentation, what it carries other than white.
Now, there are excellent type birds in all recognized varieties, and when breeding in a new variety, you want to use the variety that brings in the least amount of traits that will need to be bred out. So, if you are wanting to work on mottled, or chocolate/khaki, or paint, or lavender (now recognized), breeding to black is/was the desired choice. But you need to be using blacks with good type, otherwise you may get the variety into the breed, but the type won't be there.