And for a lot of reasons.
There's the fact that it's an important incident, of course, and ought to be celebrated worthily. There's a Jewish tale about a king giving way on the highway because a wedding party was going the other way. The Domostroi, describing a proper wedding for an upper class couple, uses the terms "the young prince" and "the younger princess" to refer to the bridegroom and bride.
As a consequence, it's also a chance for conspicuous consumption and showing off. In Florence, the bride, not quite married, could therefore be paraded through the streets to her bridegroom's house, showing off her finery without enveloping her in a married woman's proper cloak. There have been many moralists and preachers decrying too extravagant expenditures on the weddings, in many, many, many lands.
On the more practical side, extravagant ceremonies are more memorable to the witnesses. English courts accepted testimony from people who had seen the bridal procession to church when the question of whether a couple had married was raised. A great wedding feast, with lots of guests, would give them something to remember, for years or even decades. This is very important in eras where literacy and writing materials are rare, or non-existent.