The choice of hymns to sing at the wedding tends these days to be fairly limited. Perm any three from Jerusalem, All things Bright and Beautiful, Morning has Broken, Give Me Joy in My Heart, Love Divine and Lord of the Dance - and you've got the selection for around 75% of them. Things get more interesting when it comes to the signing of the register. They may have a soloist or a special request... otherwise they're likely to get one of the choir's greatest hits (not that this is necessarily a bad thing).
Some weddings don't bother with the choir, but it has a dual function of adding a bit of drama to the procession and keeping the singing going in those hymns - often these days, the congregation need a lot of help. I have sung at one where the choir proved essential. The organist didn't turn up. Everyone was panicking. How would we do the bridal procession? In the end we sung something unaccompanied as she walked in - and everyone said what an original idea it was, not realizing that the organist who played the hymns was an emergency stand-in, rapidly summoned by a whispered phone call.
In case you know anyone who isn't sure about music for a church wedding, I've set up a little web page with suggestions for music and more.