It did not, somewhat to my surprise, involve squid or a Cthulhu summoning.

The Cthulhu summoning took place later that evening, at the home of the newlyweds, you see...

As for the reason for having a wedding, well, my own wedding was all about my parents showing off how much they could spend on their daughter's wedding, but I suppose that doesn't really address the question...

For me, I guess it boils down to the fact that marriage is a joyous occasion, and people generally like to celebrate joyous occasions with other people. And the more special the event, the bigger the hoopla. So birthdays might get short shrift because they come around once a year, but I have also been to milestone birthday parties that were pretty big. Marriage is (theoretically) a once-in-a-lifetime event, hence the big hoopla.

The other thing is that the celebration is as much for the guests as for the celebrants. I'm always a little disappointed when I find out some friend of mine got married on the sly (yeah, that includes you, ukelele!) because I didn't get the chance to formally wish them well and all.