What is the purpose of the letter that Tybalt writes to Romeo?

Tybalt turns his attention from Mercutio to Romeo, and calls Romeo a villain. Romeo, now secretly married to Juliet and thus Tybalt's kinsman, refuses to be angered by Tybalt's verbal attack. Tybalt commands Romeo to draw his sword. Romeo protests that he has good reason to love Tybalt, and does not wish to fight him. Romeo protests that he has good reason to love Tybalt, and does not wish to fight him. He asks that until Tybalt knows the reason for this love, he put aside his sword. Mercutio angrily draws his sword and declares with biting wit that if Romeo will not fight Tybalt, he will ... They fight, and Romeo kills Tybalt.

So basically the porpuse was to fight but that was my in-depth version.