When a ranking (atleast in their eyes) colleague identifies key areas of policy, procedure, nomenclature or other corporate issue.
The letter is almost exclusive sent by e-mail nowadays, but used to be typed and distributed in hard copy. It is meant to provide clarity of the rules, but also to publicly indemnify current and futures violators or said policy, procedure or nomenclature.
Like invoking Miranda
Rights upon criminal suspects, the memorandizer hopes to strike fear and compliance into the hearts of would be dissenters and reinforce corporate subordinations.