There are actually a lot of reasons. The first is that it was a gift from my friend, Allyson-in-Triplicate. I always liked hers and she bought me one. (It made me feel like I was in the yoga club.)
The more significant reason is that when I was taking teacher training and we learned about the idea of OM it struck a chord in me. The idea centers around all of creation being sound. OM is the sound that the world makes. As a Christian, I believe in a God who spoke the world into being, so this idea that we are all sound feels especially true to me. I loved it when I learned from a science show once that if we could hear all sound, the sun would ring like a bell.
This core idea of matter being spoken captured in a symbol feels precious to me.
Plus there is my reaction when I first saw the symbol. It was on a ring with multiple symbols strung together to make the circle. Not knowing the symbol, I saw: heart/guy-dancing, heart/guy-dancing, heart/guy-dancing... I bought the ring. (And in a bit of serendipity later gave it to my friend Allyson because it didn't fit my fingers well but did fit her.)
I like having the symbol on my car both because it gives it personality and because it has meaning to me. Sometimes strangers stop me to ask about it--sometimes from India or Nepal. It usually surprises both of us to have a conversation about God speaking the universe into being while standing in a parking lot.