yes classifieds are helpful to promote a business or we can say these are helpful in advertising purpose but not useful for E-commerce sites. http://www.falcon18.com
classified plays an important role promoting the brand in a very desired way,to influence the site by driving traffic to build up the business
Obviously, so many business classifieds sites wouldn't thrive, if they couldn't help businesses. Results depend on the classifieds site, the business/product/service listed, how you get it listed, when and why... etc. etc. A good example is a car seller's business. They need lots of listings, presences here and there. Local businesses must be present in local classifieds lists. A harder question would be: how to build a successful classifieds business model (earning from this activity)?
Classifieds are like any potential source of traffic/clients. Are you advertising something they want? Don't think you can throw up some affiliate offer and it will work. Do you have a real service/product people need? If so, then I think you have a very good chance of making classifieds, both online and offline, work for you.