They save time: stock photos are already created, post-edited and ready to be downloaded and used. They save money: stock photos, specially those under Royalty Free license (of which you can learn all about here), come at a much, much lower cost than that of hiring the services of a photographer.
What Sells Best On Shutterstock?
Some types of photos that sell best are Portrait Photography, Food Photography, Landscapes (usually with people) and fine art photography (artsy).