These words are quoted from Isaiah 40:3. It is unusual for one prophet to speak of another prophet, but in this case, John the Baptist was no ordinary prophet.
Why was John in a near desert place? The greatest revelations to the Jews came from those situations. John deliberately separates from the leaders of the Jews. They are wrong and he has come to bring in a new order. He must not be connected with establishment in Jerusalem. He has come to shake them, speaking like Elijah. John no doubt expounded these very words from Isaiah 40. He was telling them that for the Jews there was about to be a great separation. In the Old Testament there had been a mixture of believers and unbelievers; the majority were not converted. Now however, believers will be extracted and the Jewish Gentile church will be disestablished; the rest are to be set aside. Imagine how inflammatory this would be to the leaders of the Jews. Matthew gives a summary, and Luke quoting the same passage goes a little further (Luke 3:5). ‘Every valley shall be filled, and every mountain and hill shall be brought low’: the humble and lowly will be raised up and the proud thrown down. Those who realise unworthiness will be lifted up. Isaiah prophesied that many times, and all this fell to John to preach.