A monoculture is the farming of one crop in a single area. The advantages of this process are three-fold. First, a single type of crop allows for the mechanization of the various farming process that goes into planting and harvesting foods. Heavy machinery replaces the hands of people and therefore increases the amount of possible cultivated food and the speed at which it occurs. We, as a species, prefer this method of farming because it allows us to feed the large and demanding global population we have today. Without it, we could not sustain the seven and a half billion people we have today. Second, uniformity in crops means that care of these large monocultures is kept low. This means that farmers don't need to have multiple different sprays and chemicals needed to combat disease and pests. A single product can be used for every need. Also having single crops in one field prevents the outcrossing of two species to create hybrids or new species. Third, monocultures can be cultivated by heavy machinery in large single batches and don't require separation by hand. The later processing of the food can also be industrialized from both the amount of volume brought in and the lack of contamination from another crop.
There are two key disadvantages to monocultures. First, a monoculture has increased susceptibility to disease. In a field of a single plant, the likelihood of a disease event where the entirety of a crop becomes wiped out is fairly high. A single new pathogen unknown to the plant's defense systems could infect one and eventually all the plants. Unfortunately, plants do not have an adaptive immune system so when a new pathogen is present they cannot defend themselves. If every plant in a field were to be identical and susceptible to a disease, then they all die. Second, a monoculture loses landrace diversity. Landrace diversity is the diversity of crops grown in one area. Similar to disease susceptibility, a decrease in heterozygosity prevents new beneficial mutations or hybrids from developing. A loss in ecological diversity can severely impact an ecosystem that may rely on disappearing species.