It may have come as a surprise to the Corinthians to learn that Paul also spoke in tongues (and that he spoke even more
than they). The reason why he shares this piece of personal information is to encourage the others who have this gift to follow his example, using their gift only in private worship (unless they can also interpret the tongue; see 1 Corinthians 14:13).
18 I thank God that I speak in tongues more than all of you.