Offering free content can open it up to a whole new audience who would otherwise not know about it. Everybody loves something for nothing and most people will pay for quality content. A free taster of the content allows people to form an opinion and choose whether they want to purchase more of the same.
Access to free content allows the viewer to make a good determination as to whether or not they are interested. if the material presented is really good the viewer's interest is peaked enough that they would be more than willing to purchase more material. in instances where the individual may be a collector, even if the content may have been viewed for free, they would still make a purchase to start a collection.
Most people like to know what they are getting for their money. If they watch something for free on YouTube that gives them a taste for that thing they are watching and they want more. This thirst makes them hungry for more and they are willing to pay for more content.
Even if content creators gave away all of their content for free, individuals will still want to pay for the real thing because it provides them with a richer and more immersive experience. For example, YouTube clips can't replace a higher quality DVD version of the same clip. People recognize this and so will keep paying for the DVD.