My guess is that initially flowers were used to disguise smells - filling the air with fragrance. Flowers also represent creation and natural beauty so are a beautiful addition to decorations in the church, hall or temple where the marriage and reception is taking place. Colours and choices of flowers also tell us about the couple. Well that's my two pennyworth anyways. Good question, made me think about why I chose the flowers I did when I was married. Smells and colour were important I remember.