Question: Why is the ketubah read aloud at a wedding?
Answer: Rav Aaron HaKohan M’Luniel (Orchot Chayim, chalek 2, Hilchot Ketubot  p.70) discusses the custom of the chatan giving the ketubah to his bride. He contends that the reason for this minhag is because many problems may arise regarding the kedushin b’kesef (the giving of the ring to effect the marriage). The chattan thus gives his kallah the ketubah and marries her b’shtar as well. (The ketubah actually states, “Heh’vi li linto k’dat Moshe v’Yisrael – Be my wife according to the laws of Moshe and Israel.”)
Giving the kallah the ketubah in the presence of two witnesses (eidim) is an extra precautionary custom to ensure that the kedushin is kosher. It’s another form of kinyan. (I once heard that Rav Aaron Kotler used to insist that the eidim stay under the chuppa to testify that the chatan gave the ketubah to the kallah.) Accordingly Rav Aaron HaKohan M’Luniel cites the custom in Provence to recite birkat chatanim when the ketubah is given.
This may also explain why we read the ketubah at the wedding ceremony. Just like all hear the kedushin being effected through kesef, so too should they hear the kiddushin effected through shtar.