Not all vendors use benzoic acid, I know nicotine river doesn't (I DIY with nic salts). That is one reason I chose them, although their nicotine is excellent, generally. With nic salt premade juices or salt pods, though, many of them do use benzoic acid.
Nic salts pretty much provide less throat hit, which is what I was looking for out of my vape. I DIY at 18 mg/ml and I get a very smooth, flavorful vape.
Some say they are more quickly bioavailable although I haven't noticed any huge difference, if it's there. With some of the higher salt pods like the Juul at 50 mg/ml, I could see it "feeling" more bioavailable more quickly, mainly due to the ultra high nicotine content (although its delivery system is pretty weak, you won't be getting huge mouthfuls of vapor).
Unless you are looking for a smoother throat hit, I'm not sure you need them necessarily. If you are struggling to get to a higher nic level without a large throat hit, I could see where they might have some use.
Anna