There are a few nice features of writing vs. talking in person. First, it makes it less likely two people will take what you said differently. Second, it immediately creates a shared vocabulary. Third, it creates an artifact that can both be revisited and shared with new folks entering the company. Fourth, everyone knows everyone else knows. Even if what you're saying is totally innocuous, if you tell Person A but Person B finds out you told Person A before you can tell Person B yourself (maybe they were out sick that day, whatever), you run the risk of people seeing an "inner circle" and an "outer circle." "Knowing first" can become a kind of very counterproductive gold star.
I think those are more like the reasons he was advocating for writing rather than some obsession about process per se.