It’s easy to imagine that the uniforms the bride-to-be chooses for her bridesmaids are created in order to make herself more dazzling by comparison. That may be true for some brides, but it’s not where the tradition started.
Beginning around Ancient Roman times (when the idea of a bridal party first arose), bridesmaids would not only dress like each other, but also just like the bride, covering the altar with nearly indistinguishable ladies. And that was the point: the bridesmaids were decoys.
In the early days of the ritual we recognize today as the wedding, brides had a lot more than floral arrangements to worry about. Any glad tidings had the potential to attract evil spirits, for one thing—and then there were all the would-be grooms the bride had turned away. So to keep the bridal couple safe from demons and angry, rejected men, they dressed their friends in matching wedding attire. The theory was that the bevy of brides would confuse any malcontents long enough for the happy couple to get their vows on.
The threat of demonic wedding crashers had petered out by the Victorian era, when brides began to relax their protocol and dress more elaborately than their bridal parties. Unfortunately for some modern bridesmaids, the uniform stuck.