Why are dictionaries called dictionaries?

One could argue that dictionaries are called as such
because they tell the user how to say things. Or you
could say that the Latin word dictio means ‘a word’,
and so a dictionary might be seen as a compendium of
words.
In fact, the word ‘dictionary’ (in its Latin form
dictionarius ) appears to have been coined by the
Englishman John of Garland in the early 13th century
as the title of a children’s textbook written as a guide
to Latin composition, and Garland makes clear in his
introduction that he is thinking of dictio not so much
in its sense ‘word’ but in its sense ‘connected
speech’, because by using his guide the learner will
be able to put words together to form connected
speech.