In self- pollination, there is no diversity in the genes and therefore the purity of the race is maintained.
The plants do not depend on external factors for pollination and even smaller quantities of pollen grains produce have a good success rate in getting pollinated. In the case of cross-pollinating plants, there is a threat of the entire crop dying out if there are no pollinators to facilitate the process. The disadvantage of self-pollinating flowers and plants is that there is little genetic diversity because the same genes are passed down from the parent plant to the offspring.