Suppose you are working as a farmer, and you want to grow corn - it's the only crop you want to grow. Of course, you'd have to make sure there isn't anything else staying with that corn, right? But why? There are many reasons as to why a farmer would choose to use a selective herbicide. One is that it reduces competition. Plant species living in the same area and using the same resources are going to compete for those resources; hence, no matter how much water you give to your corn, if there's something else using it, then your preferred crops aren't going to get enough of it. This may also determine the quality of your corn, and the amount of yield it provides. Sometimes, you might also want to get rid of annoying plants that get into your way of harvest as well. Killing a certain plant species also ensures that you have enough space for your preferred crop, as well as guaranteeing that unwanted plants won't disperse their seeds around the area and take up more space in the future.
