The burning cars are used as blocades against the Basij and supression forces. I read that one person was advocating using them as moving bombs by stting them on fire, and then putting them in gear, and rammimg them into Hardliner forces.
No doubt adding extra bottles of gas in the back seat first.
I imagine a burning car would make a very effective blockade. They burn hot, are heavy (so you can’t just push them out of the way), and produce a lot of very nasty smoke. All of which is good for confusing an enemy. Especially if you are on foot and they are in vehicles. Just brew up a few old cars and you level the playing field. A pile of tires works even better, but you won’t be able to find a few hundred tires as easily as you can a few old cars.