If you've ever been a bridesmaid in a large wedding, you know for sure that you don't ever wear the same dress again. What the question refers to is multiple bridesmaids wearing matching dresses to that wedding. Or at least dresses that match in color. And at it stands now, those dresses are traditionally unsuitable for any other occasion and don't look anything like the bride's dress.
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.
There's a superstitious reason for the tradition of having multiple bridesmaid that look alike, which you can read about at Mental Floss.