Words that are written have a longer lifespan than words that are spoken. That something must be written down indicates they may never be forgotten, and thus are important. See also Revelation 1:11, Revelation 1:19; Revelation 14:13; Revelation 19:9.
5 And he who was seated on the throne said, “Behold, I am making all things new.” Also he said, “Write this down, for these words are trustworthy and true.”