What exactly is the case? What does he hope to gain?
Ramban (on Pasuk 13): After betrothing and marrying her, he did not enjoy the first time he is intimate with her and claims before Beis-Din that she committed adultery after the betrothal, 1 in which case she is Chayav Sekilah, in order to exempt himself from having to pay her Kesubah.
Ramban: Otherwise the Torah would not write "Liz'nos Beis Avihah" - See Pasuk 21. See also Sifsei Chachamim on Pasuk 20.
Why does the Torah insert the word "ha'Zos", implying that she is in Beis-Din, too?
Rashi: To teach us that one can only open a case in Beis-Din in the presence of the defendant.