Pedestrians absolutely pose potential harm to a car (though, of course, the car poses far more potential harm to the pedestrian), especially with all of the crumple zones and sensors. Even a relatively low speed collision of say, 25 mph, I'd be surprised if the car is getting away with less than $5k in repairs.
Jaywalking is dangerous, especially on busy streets. And I admit to my fair amount jaywalking when I worked in downtown Chicago.
The horn is primarily about expressing anger but about imminent danger. A pedestrian steps out, from limitted visibility between cars, into traffic they deserve to be honked at. Not because of anger, but because they are putting themselves and others at undue risk.
That said, I don't think Ive ever personally seen someone respond to a horn in a fashion to avoid an accident. Twice last year, I was involved in accidents caused by someone else changing into my lane. Both times I saw it coming, laid on the horn and braked, but they kept coming faster than I could stop.
One, I couldnt avoid at all. We were in a construction zone, and the car next to me just kept coming over. I saw it coming, but had no where to go. No shoulder, just a concrete K-rail with about 6 inches of wiggle room. Side swiped me at about 55 mph. Entire driver's side was fuckedd up. Couldnt even open my door. Also managed to fuck up some of the passenger side as I did everything I could to get out of the way. (little bit of fender damage, cut both tires and scraped up a wheel)
The second one was I was in a left turn lane and the dude tried to force is way in. I didnt even see his signal until it was too late. Again, laid on the horn and brake, still mamaged to put his rear door into my passenger side front quarter panel.
Had a third actual miss earlier this year. Was driving in the 2nd from left lane in a rain storm, doing around 80mph on the highway. Car in the left lane, without signaling, starts coming over. Again, slam on the brakes and the horn, asshole keeps coming. Went from 80 to 60 in about a second or so, nearly getting rearended in the process. My car also started going sideways. In a lesser car, I probably would have lost it, but mine is pretty stable. Soon as I let hard off the brake, it recovered easily.
Point is, everyone assumes if someone is honking they're road raging at you, and some surely use it that way, but it's primarily about signaling imminent danger, and you should really take note (and probably return to your lane until you can figure out what the danger is by checking your blind spots, etc).