Things don't have meanings all by themselves; they have the meanings you give them.

Thank goodness, there is no such thing as a Great Global BDSM Overseeing Authority Figure who prescribes what we are supposed to find meaningful or not.

Many people use a collar as a symbol of commitment, ownership, love, dedication, Dominance/submission and so forth. For many it can also be an item of aesthetic pleasure. It is close to the head, so it is within view when people communicate face to face. Putting it on someone involves messing around in the comfort zone close to their head, with all its sensitive organs and carotid artery. So allowing someone to put a collar around your neck is an act of trust.

Personally the symbolism has never caught hold in my mind, and quite possibly it never will. For me, a collar is empty of meaning. So if I used one, it would just be another restraint. Very practical, of course, because it goes on a narrow part of the body where it can't be removed if locked. But as personally I don't think collars look very good on men nor on women either, I have to decline for aesthetic reasons too.

It frequently happens in stories that a character leads another character from pillar to post with a collar and leash, even though the thusly led would obviously follow anyway if so commanded. I tend to find this a source of amusement. Unfortunately, the amusement at times throws me out of the story, and I need a while before I can take the characters somewhat seriously again...