From The Atlantic, on why Dylan included a song about John Lennon on the new album Tempest:

Because Dylan deals in myth. It informs his output as much, if not more, than the actual people in his life. Dylan uses motifs like floods and trains in order to wrestle universalities down and trap them in his verses. He, like Lennon, will always be associated with the 1960s, but more than any other major artist from that period he has always written songs that are designed to transcend the context in which they were created.