FLux keeps oxygen away from the metal that, when you get it super hot to solder something, would easily oxidize if you didn't keep it away with the flux, which could prevent the solder from flowing properly. Plumbing these days requires new versions than those of old, which are easier to burn adn mess up your joint. So, if you're very practiced, you can use any flux and make a good joint. I find that using a tinning one gives you that extra indicator that says the joint is hot enough so you can add your solder. That's not obvious with a non-tinning flux, so you might overheat the joint. As to required, nope. On a heavier valve or fitting, it's easier to overheat, or not get it hot enough, and the 'tin' in the solder gives you a good indication of when it's ready.
