1. It unleashes one's vanity---especially the well-tinted ones. Even for just a second, the person would glance at the car window to check how he/she looks for that moment. As one's hair flows with the polluted wind, the image that the car window reflects can give an immediate sense of satisfaction.
2. It urges one to commit temporary vandalism---especially the dusty ones. There is that excitement of being caught by the car owner and there is the relief that it will soon be washed off. People tend to get the urge to write and the popular ones include the male genitalia, their frat name or logo, and their own names.
In relation to the second item, I saw this dusty van parked in front of our office. And the one who committed the crime seems so angry yet utterly concerned. It seemed as if the van spoke to him and gave him the courage to send its message.
You can see that "AKO" is bigger, perhaps, stressing that the message came from the van and not from the person who wrote it. And it's anger is so obvious not only with the three exclamation points after the bad word but also with the follow-up "Ka".
Unlike drawing the male genitalia (for purposes that women like me will never understand), this temporary vandalism can yield to something good---an immediate car wash. Thus, giving car wash employees another customer which would yield to better income and a better economy.