Why does Chatter need for TextPairs?

Question: What are TextPairs for? I read text pairs description and do not understand why you divide a sentence in such a way.

Answer:
I divide a sentence into: words and phrases.

But in order to remember "how correct natural language sentence looks like" I also
remember typical text pairs.

Text pairs are like the links in the chain.

TextPair table will help TextWriter to build sentence in a correct
way.

Please, ask me more if it is not clear.