I've had this conversation with vanilla friends before: What is with collars? Why would you ever want to be owned by someone?
Look, if you've ever felt different, unaccepted, or unwelcome, you know how powerful feeling like you belong can be. Many kinksters grew up knowing that they were deviant, knowing that the things they loved and longed for were different than what the world was telling them they should. Many of us grew up with our family, our friends, the media, and doctors all telling us that, because we had these desires, there was something wrong with us.
That we were, by nature of who we are, unlovable.
Many of us still live in a world that, if these things about us were publicly known, would disown us. When this part of our lives, of ourselves, is put out there, we often lose friends, family, jobs, standing, safety, and even our lives.
So, yeah, having someone willing to claim you as theirs, to give you a symbol that says that they love and accept you as you are--because you are who you are--that's really powerful for many of us.
Personally, I have collars that I've made or bought for myself. A sort of symbol to myself and to the rest of the world that I am who I am and I accept and love this part of me.
Symbols are powerful and every culture and group has them; this is just ours. And, even if it doesn't mean anything to you or even if it means something negative to you, it's important and meaningful to us and deserves respect.