Sugar is hygroscopic, a bit. A little sugar enhances browning, too. It is just for taste in most things (takes some of the "bite" out of the salt), but it is essential for fermented sausages like salami and even summer sausage. It feeds the bacteria that produce the lactic acid. Without sugar (glucose or dextrose rather than sucrose) the Lactobacillus don't take off, it takes too long to get the pH down and you run a real risk of really bad bugs (Salmonella, Listeria, E. coli) growing instead. It can have chemical effects, also. I used to make jerky with a 50:50 brown sugar/soy sauce marinade, and if you leave it in too long, you get really dark, caramelized, brittle jerky with almost no meat texture left to it. With only soy sauce, it just gets saltier.
The problem I am having is my wife is allergic to corn, so dextrose is out. I need to find a non-corn alternative.