Not everyone could write -- the secretary wrote for them, transferring their thoughts onto paper (or parchment).
Not everyone could write well -- the secretary cast their spoken words into clear language for them.
Not everyone could write legibily -- the secretary wrote (generally) in a clear hand which others could read, even where the language was different.
Some secretaries were also companions, chroniclers, factotums and body-servants for their patrons. Some even, over time, came to serve more than one patron, as their fame grew or fell, and their patrons' wealth fluctuated.
The Secretary of the SCA
Many SCA colleges may find that the mundane university bodies require them to have a secretary within the college if they wish to be affiliated to the university. In the game, the SCA does not have any officer that can be directly linked to the modern role of the secretary so commonly one of the known officers (eg. the lists officer, chronicler, arts and sciences officer) is listed as the secretary. This may be explicitly written into the constitution of the college.