To make it impossible for Juliet to marry Paris later...and to make it plausible that Romeo would not want to kill Tybalt. I doubt very much that Romeo actually held as much affection for Tybalt as he pretended, but more that he was not willint to duel with a member of his own family. Likewise, if Juliet had not been married already, it is unlikely that Friar Laurence would have been willing to take such drastic measures to keep Romeo and Juliet together.
