A Brooch is a sign of rank usually, romans and vikings used them alot, to indicate they were freemen and not slaves and to indicate their rank as freemen. So maybe it is just a sign of freedom. Broaching has different meanings tho, broaching a new subject, cutting materials at an angle, coming to a dead stop in sailing and attacking in multiple ways.
ie as a tactic in combat. It can also mean breaking through, broaching a wall. (breeching)
But it sounds like you ment Brooch, ie the object that either hangs around your neck like a neckless or is pinned to your shirt as a decoration. I think they were originally signs of freedom. ie of not being slaves.
""Fibulae were shaped somewhat like a large safety pin and were used to hold clothing together. They came in many varieties and held prominent significance for the identity of the wearer, indicating ethnicity (until local costume became Romanized) and class. Elaborately designed fibulae were an important part of Late Antique dress, and simpler ones were part of Roman military equipment.""
So whilst we do use them, they aren't a scottish only thing, but have been used by people all over europe/arabia for thousands of years. Probably by anyone who wore clothing really and didn't use a rope as a belt.