How common is it in the USA for strangers to ask if someone needs help when they see them doing something?

Depends where, but overall quite common. The other day 3 people pulled over to help me give a dog wondering the streets water. I brought her to shade, someone else brought a bowl, someone else had a water bottle, and the last person happened to have some shredded chicken. Like, this is for a dog. Best people story I have was when my dad was rear ended while stationary on the freeway because the little starter battery in those newer cars died on him in traffic that was there because of a different accident. So basically everyone who was helping the old accident had to catch the next freeway exit to come back to help him, the police and tow truck. Someone not paying attention smashed into my dad at 75 mph and totaled his car with the the rear wheel flying off the other side of the freeway and smashing a Tesla’s frunk and cracking the bottom of that random dude’s windshield. Whole freeway shutdown, etc, so the people stuck in the shut down rushed over to each car and started trying to help everyone as much as they could before the ambulance and whatever else arrived. Around 14 people running around with first aid kits and tools to clear a path to everyone involved in the crash on the freeway where 3-4 lanes were blocked by the cars and debris. I have pictures of the car. Thing’s back end and right side were smooshed or straight up across the freeway lanes