Obtaining the right to exclusively sell a product or service
(aka having a monopoly)
Inventors are encouraged to invent, in part, if they can make some money from their inventions.
A patent gives an individual the right to exclusively profit from his invention for a certain number of years, allowing him to recoup his investment and make some money.
Why didn't Fulton just apply for a patent?
The first United States patent act was not enacted until 1790. To obtain the exclusive right to profit from an invention before 1790, inventors had to apply to each state legislature for a monopoly in that state.