Personally I never understood the concept. I’ve never given one, nor ever would.
Because you’re attaching the idea of loyalty and commitment to an object. I rather show the girl through my actions that the things I tell her are legitimate.
Plus I’ve seen many couples end in a train wreck and they just so happened had a promise ring in the equation. If you ask me, if you’re gonna go to that length, just wait for the real thing aka the engagement ring.