According to PerfectWeddingGuide.com,
The bridal gowns were torn by guests attending the wedding.
Of course, nobody wants to tear off a piece of their gown. Therefore the garter separates were made. Dresses are now kept intact, and nobody is bothering the bride by tugging at her dress.
Traditionally, the witnesses would accompany the newly married couple to the bridal chambers to ensure consummation. The wedding garter being retrieved from the bride served as proof to other guests that the deed was done, and the garter was then tossed into the sea of people.
Symbolically, the garter was thought to influence fertility. It also symbolized marriage consummation and future children for the couple. Because of this, the garter was fiercely sought after at weddings. It was an honor to detach the garter from the bride and was believed that the luck of the wedding couple would pass to the guest who obtained the garter.
Aren’t you glad times have changed!!! The bride now sits in the center of the dance floor for the groom to retrieve the garter and toss it to the bachelors attending. The garter goes above the knee for easy access and keeping the tradition more modest.