I'd guess that it's to break the acid taste of the vinegar. But that's just to ameliorate the childish palates of the average USian consumer. And yes, it is a sugar substitute, but not having tasted this particular brand, I can't comment on its flavor.

And Ranch dressing is probably the most common dressing in the US, with the possible exception of Thousand Island, which, AFAICT, is a mixture of tomato ketchup, mayonnaise, and pickle relish, sometimes without the relish. Lousy stuff.