When someone is about to leave us (e.g. death or a departure to a faraway land), the last words they speak are ones that they consider to be the most important. There’s no time for wasted words when you have little time left with your loved ones. You want to utilize what time is left to impact them and, as we see in the case of Jesus, to leave any final instructions.
Among the last words spoken by Jesus after his resurrection and before his return to heaven are the words which have come to be known as: “The Great Commission”:
Go into all the world and preach the gospel to all creation. Whoever believes and is baptised will be saved, but whoever does not believe will be condemned. (Mark 16:15-16)