Acts 15:27

‘We have sent therefore Judas and Silas, who themselves also will tell you the same things by word of mouth.'

In order to ensure that there could be no doubt about the agreed situation they were sending Judas and Silas, so that they would not only have the letter, but would hear by word of mouth all that had been said and agreed from the mouths of elders of the Jerusalem church. There could be no tampering with a verbal witness, especially such distinguished ones, and it would bring home the message more really and personally.