I guess this is it! It's got a thing to do with the specialty of the block and maybe some other reasons unknown to us now but, won't totally be for the fun of it. The message itself could be another unique way to mark the block and who knows, it might not the message be attached with some sort of importance in the later future. We all know how some art antiques are this days and with blockchain technology evolving to almost every sphere, someone might has seen an opportunity in this for which the rest of us are yet to know off or attach some value.