A cleaner sample if I use Phenoxylene Plus ...but why?

Advertisement for Fisons Pest Control Limited. In the upper right is an image of a farmer's hands sifting grains. Drawn with a thick outline and blocks of parallel lines. On the left and lower margins is text promoting the properties of Phenoxylene Plus, an agricultural fertilizer / herbicide, in the form of a dialogue between 'Harry' and 'Jack'. Text in the lower half is bordered by a 'cut out' box. Paper cut to margins. Selected text from advert, 'It's Fisons for good farming', 'hormone weedkiller', gives a cleaner sample from maximum yield', ' Phenoxylene Plus proved on every type of land' and 'Talk to your local Fison's Technical Representative'