If you add a little light color like yellow for example to dark violet, you will barely notice the change, because the violet is a much darker pigment than yellow. To see a vast difference in the color change, you will have to add a lot of yellow (lighter pigment) to the violet (darker pigment), and also waste a lot of unnecessary paint. On the other hand if you add only a little bit of violet to the yellow you will notice the difference immediately. The other reason is that you can achieve greater transparency in your washes, if you add little bits of dark color to a lighter color, rather than adding lots of dark color to a light color and ending up with muddy colors.