For drivers slowly meandering a little too close because they're not paying attention, or the dude with the nose of his car stuck too far out into the lane as he's inching forward to make a turn as you pass, then the open palm smack is great. It's loud and catches people's attention, making them realize "Hey, somebody's right there!"
For drivers who intentionally squeeze me into parked cars, pull halfway through an intersection in front of me only to be blocked by stopped traffic forcing me to slam the brakes to keep from broadsiding them even though I have a green light, impatient drivers who pass me at mach speed only to cut me off making a right turn 20' later, etc... These are the ones that get a cleat to the paint. The point I want to make isn't just "Hey, somebody's here!" They know I'm there and just don't give a crap. The point I want to make is "You tried to kill me with your car, prick. Here's a reminder to give at least an arm's length of clearance."

And yes, if you hit a car with your fist, you can dent it. I also don't suggest it, because you might hit a support beam and break your hand. (The relatively low-speed and low-force impact of a punch usually forms a dent that can be pulled back out with a plunger, causing no lasting damage to the vehicle.) But still, don't do it because you might break your hand.