Why was it such a valuable gift to preach in a language everyone understood?
It was valuable because the congregation needed to hear and understand the Word. Since many could not read Scripture at home, they depended on hearing it from men's lips. Preaching in a common language allowed everyone present to receive the teaching, whereas an unknown tongue left some without understanding.
Les explains that in the Corinthian congregation most could understand Greek, but some could understand only Hebrew or Aramaic. Paul therefore said it would be great if they could come into the congregation and teach and preach in a language they could all understand. That was a great gift, because people needed to hear the Word; they could not go home and read it, so they had to hear it from men's lips. Les also compares this to missionaries today who must attend language school for four or five years before going to the field, illustrating how valuable it is to speak in a language the people understand.