To properly use a planer, one face of the wood must be flat and out of wind (not twisted) before you start. So the first step in planing wood is face-jointing. You can do this with a hand plane, but most of us prefer a jointer for the task. Curiously, most planers on the market are 12" wide or more. On the other hand, most jointers in home shops are 6" or 8" wide — with the 6" version predominating. Face-jointing a 12"-wide piece of wood on a 6" jointer is a pain in the woodshop. The same is true of 8"- or 10"-wide stock. It can be done, but if it were me, I’d just rip the stock narrower and go from there. Imagine if you had a 12" jointer to go with your 12" planer ... life would be a bed of roses (or at least of woodchips).
Twelve-inch jointers are expensive, and even the benchtop models of planers are not cheap. But by combining both tools into one, you have a more practical stock-surfacing system at a price that can be very competitive. Do you need to face-joint 12" stock regularly? Perhaps not, but what about 8" or 10"? If your answer is “yes,” a 12" combo planer/jointer may be in your future.