The officiating rabbi at a wedding, known in Hebrew as the mesader kiddushin, has two primary functions:
The first and primary function of the rabbi is to ensure that the entire wedding is properly conducted according to Jewish law and custom. He makes sure that all the "paperwork," i.e. the ketubah (marriage contract), is in order; that the marriage witnesses meet all halachic criteria; and that all other parts of the ceremony conform to halachic standards.
Secondly, there is a blessing that is technically the groom's responsibility to recite before placing the wedding ring on the bride's finger. Since the groom will likely be too nervous at this juncture to properly pronounce the blessing, the rabbi recites it in his stead.
Please visit our Wedding & Marriage Section for more information on Jewish weddings.
I hope that I've been helpful today. Please feel free to reply.
Rabbi Menachem Posner