What is the purpose of apple cider vinegar in vegan recipes?

I have La Dolce Vegan and Vegan with a Vengeance, and I see a TON of recipes that call for one. One that I'm looking to make is a cerole with a cornbread topping that includes said ACV. I was just wondering if anyone knows what would happen if i left it out or added cream of tarter/bakingsoda/baking powder and a little more liquid.