Why do we park a car?

Because, of course, we used to park a horse. A long time ago, horses were taught to "park out" - to stand with the hind quarters behind them. This was done when you needed the horse to stand, because it was harder for them to move rapidly from that position.

This also became something some people did to make the horse's back lower for mounting. (We now know it's very bad for the horse's back to do this but, of course, some people still do).

Only gaited horses are generally parked out these days - when standing for inspection at a show.

Or, of course, you can just park your mule...