Hi Daido,
Well, there is a modern and a historical reason. The main modern reason, in America anyway, is the influence of the Harada-Yasutani Lineage (through the Maezumi Roshi Lineage and others) who combine Soto and Rinzai Practices (I joke that they are "SINO's" ... SOTO IN NAME ONLY). They are a small group in Japan, but represent perhaps the largest body of Zen Teachers in America. They face into the room in the Rinzai Way.
Traditionally, Soto Zennies would "face the wall". I actually think it is better for less experienced sitters to do so, as it reduces the sensory stimuli, thereby facilitating calming the mind.
Perhaps I should do so more at Treeleaf Tsukuba, but our physical layout does not allow it for the middle section. Also, I believe that the sitters' "looking downward toward the floor" also reduces sensory stimulation, so the effect is about the same. For more experienced sitters, I do not believe that it matters ... and, in fact, we should develop the ability to sit anywhere, however noisy, busy or distracting.
I was surprised when, a couple of years ago, I conducted an unofficial poll among teachers who are members of the Soto Zen Buddhist Association of North America, and found that most of the Soto teachers seemed to be open to sitting either way.
Anyway ... the historical reason may be a mistranslation of Bodhidharma, regarded as the First Patriarch of Ch'an or the Zen tradition, and a writing long attributed to him (
The Two Entrances and Four Practices) that used the term in Chinese "biguan/pi-kuan". Historian Heinrich Dumoulin discusses Bodhidharma's wall-contemplation.
"In an ancient text ascribed to Bodhidharma, his way of meditation is characterized by the Chinese word pi-kuan, literally wall-gazing or wall-contemplation. Except for the word pi-kuan, the same passage is found in a Mahayana sutra; it reads: "When one, abandoning the false and embracing the true, in simplicity of thought abides in pi-kuan, one finds that there is neither selfhood nor otherness, that ordinary men (prthagjana) and saints (arya) are of one essence." (Zen Enlightenment, p. 38).
The actual meaning of "wall gazing" may not be a literal "sit while gazing at a wall", but closer to "sit as if a wall seeing". Nobody really knows what the term originally meant however. The great Zen Historian Yanagida Seizan has (ala Shikantaza) interpreted the term to denote a sort of witnessing of the world with the steadfast detachment of a wall in which one “gazes intently at a vibrantly alive śunyatā (emptiness).”
So, whether facing the wall, or away from the wall ... just sit, without thought of in or out.
Gassho, Jundo