In order to garden successfully with plants growing on a wall, it is important to recognize the difference from traditional gardening. I never thought about gardening on the ground or in pots as gardening horizontally, but that is exactly what you are doing. Of course there are variations in the slope, but by and large, traditional gardening has been horizontal. Succulent plants are often found growing on cliffs in the crevices of rocks or on the cliff edges, clinging to what soil they can find purchase in. This ability to eke out a living in little soil makes them pretty good candidates for vertical gardening.
"Succulents are the conservationists of the plant world"
— Robin Stockwell