Yes, on reflection - now I see (realize) what you mean.
When I've seen joiners used, they're used to square the edge
(vs. the surface) of the wood stock. Conversely, a planer is used to smooth/even the much wider, surface of a board. I can see (visualize) now how these two pieces of equipment couldn't be used interchangeably to perform these two different functions.
And the specific nature/function of the equipment answers the question about "vertical" vs. "horizontal" - which really is in reference to the relative mounting/orientation of the cutting blade (Planer = Horizontally | Planer = Vertical).
It still seems like there would be other, easier methods for squaring a board edge, without the need to invest in a joiner - like a tablesaw? Maybe there's some drawbacks to this . . . but if so, those aren't readily apparent to me (in my still novice wood working capacity.