Some products, such as antioxidants, provide health benefits that are important but not easily verifiable in the short term. Others, such as dietary fiber, glucosamine, probiotics and prebiotics (good bacteria and the fibers that “feed them,” respectively), often are purchased for fairly immediate relief of symptoms.
I personally have a strong belief in the healthfulness of certain nutraceuticals, pre- and probiotics being two. A 2005 Consumer Reports article entitled “Probiotics: Are Enough in Your Diet?” notes the best support for these organisms' benefits involve gastrointestinal relief, reduced allergic reactions (atopic eczema), respiratory infections, reduced urological infections and risk of colon cancer.