So sometimes in some situations we have to use chemical methods instead of mechanical ones. This is usually for 2 or 3 reasons. Firstly the cost reduces dramatically. Secondly by not disturbing the soil, no further weed seeds are brought to the surface, thus risking further contamination, and thirdly, in some situations, like lawns for instance, it it not practically possible to remove an infestation of weeds by mechanical means without seriously damaging the lawn itself. So we use a product which kills broadleaved plants, but does not harm grasses.