I only bring it up because he showed up and got rid of the tree where the secret texts were stored, in reality Rey had the texts, which in theory could allow her to continue the Jedi tradition and teach others, which Finn found on the Falcon. I think it was a subtle way of Yoda telling Luke that it was time to let the past go and to pass the torch allowing the Jedi order to evolve under the influence of Rey, in turn this would allow Luke to become one with the Force and become a Force Ghost. Aside from the fact that it was a way of getting rid of a beloved character, I think that it was Yoda's way of telling Luke that his era has passed and to let go in order to make way for a new era led by Rey, it just wasn't executed well.