Backpack sprayers are extremely useful in a variety of horticulture and agriculture applications. They're typically used for medium-duty spot spraying of fertilizers and pesticides in home gardens, lawns and small orchards.
Unlike handheld sprayers, backpacks provide the added benefit of not having to carry the sprayer around with you by hand. This is why they are such a popular choice among professional lawn care contractors who are spraying yard after yard, all day long.