Everyone makes a point to dress well for a wedding of course, since most weddings require either a suit or a dress or some culturally-specific attire. Most weddings are either semi-formal or black-tie events--as would be stated in your invitation to prevent anyone from arriving in jeans and a t-shirt to a semi-formal dress shirt and tie event and stand out like a sheep among wolves.
photo courtesy of: http://agirlgottaeat.files.wordpress.com/2008/05/1280x1024-satc-1.jpg