If you work with rough lumber, a planer will be absolutely necessary, except for the most rustic of projects. Every piece of rough cut lumber ends up somewhat not straight, not flat and not consistent in thickness, either from variations during the sawing or from stresses which occur while the wood dries. The planer, combined with the jointer, is a one-two punch to remove these variations and produce straight, flat and consistently thick lumber. The reason the planer is ahead of the jointer on this list is that some lumber is straight enough and flat enough to plane without jointing if the job is a little less finicky, thereby skipping the jointer.
As the comic George Carlin might say, a shop is mostly just “a place to put your stuff.” The physical aspect of a shop is indeed simply a space of some kind — a garage, a barn, a teepee — that keeps the rain, rust, and robbers away and houses a collection of implements needed to saw, plane, slice, sand, and pound raw wood into useful objects. But just like the difference between a house and a home, what transforms a building full of tools into a comfortable workshop goes beyond the mere physical aspect.
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