Genesis 30:34 - Joseph Benson’s Commentary on the Old and New Testaments

Laban was willing to consent to this bargain, because he thought, that if those few he had that were now speckled and spotted were separated from the rest, the body of the flock, which Jacob was to tend, being of one colour, either all black or all white, would produce few or none of mixed colours, and so he should have Jacob's service for nothing, or next to nothing.