There is something very pleasing about a traditional invitation. It evokes a civilized feeling - Downton Abbey comes to mind. Even if you work for the Crawleys, for this one day of your life you can live as if you were one of them.
These invitations signal your guests that they are in for a lovely time. There probably won't be any big surprises - unless you decide to arrive in a carriage drawn by four white horses. Your guests will know that they are expected to dress up; no one will show up at your wedding in blue jeans. Tastefully conveying your expectations for your wedding is a gracious way to invite your friends and family.