The jointer is the first step in converting rough wood stock into usable pieces that are flat, straight, square, and the correct size. It is used to make the face of a warped, twisted, or bowed board flat. After one face is flat, then the jointer can be used to straighten and square an edge in reference to that face.
Hardwood lumber yards sell wood that is either rough or surfaced. Rough lumber comes directly from the sawmill, where it is cut with giant band saws and circular saws that leave a very coarse surface. Wood is also a natural material that has internal tension and reacts to ambient moisture, so it is always moving. Buying rough lumber and surfacing it yourself gives you the best control of your final product. Building projects with truly flat, straight, and square pieces of wood is vastly easier than trying to shoehorn together warped and twisted pieces, and you will save lots of money by doing the surfacing work yourself.