That’s What It’s for?
This is exactly why labels are important. I own a few dresses and shirts with this type of clasp, and I had no idea what it was for. It turns out it’s supposed to keep the bra strap in place; I’ll have to try it next time I wear one of those items. It’s such a thoughtful thing for a designer to add if that’s indeed its purpose.
In any case, if it is effective, then that’s great! Maybe the symbiosis between the two garments helps the dress to stay in place. Even so, I am not confident that this would work with every bra since they are all designed differently – especially concerning strap placement. If none of my bras fit perfectly in the right spot, it would just be my luck.