Most woodworkers know that it takes both a planer and a jointer to get the most out of raw wood (at least for power tool users). The joiner is used to flatten one face and square an edge and the planer is used to make the second face flat and parallel to the first. For more information on this process, see this video on Sawn Timber Milling. Since these tools are so expensive, it's not surprising that most of us can only afford one tool at a time.
So deciding which one comes first could make the difference between having a useful new tool in your store and having a dust-gathering decoration in the corner until your partner arrives. Whether you create an eye-catching piece of furniture or preparing a rough board to finish carpentry work, starting with a board of uniform thickness sets you up for a good result. A wood planer helps you unify a piece of wood into a board with the exact same thickness everywhere. A properly planed board is completely flat on both sides, eliminating rough spots or excess bark.
Handheld portable planers are useful for off-site applications, while stationary planers work well in woodworking shops and large-scale operations. A jointer is used to make the face of a warped, twisted or arched board flat. After the boards are flat, the jointer can be used to straighten and square the edges (the protector for the photo is removed). I have had to explain this to many people new to the world of carpentry in my local carpentry shop who are confused as to the use and function of a “joiner” and “planer”.