When was the last time you saw Jesus, in the flesh stand before a group of people and teach them his gospel and give them some guidance about our day, in our day? You have not.
That is the duty of the prophets of God, who from the beginning of the earth, have received revelation from Jesus Christ himself, and then stood before the people and taught them what Jesus wanted them to do in their time period.
Now most of the gospel message, I agree transcends time, but there are some very important instructions given for the relevant events that are taking place in the different time periods that need particular attention and guidance, which is why it is a benefit for Jesus to have a prophet ready to declare his words of in the flesh and particularly 'in our day'.