Do salespeople serve a useful role in society?

Yes and No. Although on the one hand, salespeople don't contribute to productivity in traditional terms (in other Words they don't actually make anything), they do provide a service which potentially generates revenue (for themselves and their company) which in turn raises more tax revenue and increases state income.

Clearly the main job of salespeople is to sell more products, but in doing so, they also increase awareness of the products they are selling which consumers may otherwise not notice. Having said that I would also say that the benefits of salespeople are secondary and that the world would be no worse off without them (and in many cases better off in situations people are convinced to buy something they can't afford and don't need). I should add that I'm not a salesman - had a very brief stint as one before walking out of the company because I found the high pressure and unrelenting environment so unpleasant.