According to master Houyhnhnm, what is the purpose of speech?

Here is a quote from Part 4 Chapter 4

"He argued thus: "that the use of speech was to make us understand one another, and to receive information of facts; now, if any one said the thing which was not, these ends were defeated, because I cannot properly be said to understand him; and I am so far from receiving information, that he leaves me worse than in ignorance; for I am led to believe a thing black, when it is white, and short, when it is long." And these were all the notions he had concerning that faculty of lying, so perfectly well understood, and so universally practised, among human creatures."