I think they're hoping people will see the ads and decide they need the drugs. They don't know anything about the drugs except that the commercials say they work. Then they ask their doctor about it and next thing you know big pharma has a new customer. The doctor gets something in return if he signs up enough people for their drugs.

Big pharma sends out lots of sales reps to doctor offices and they give them free samples of drugs they can prescribe to their patients. The drug commercials are a constant reminder of how great the drugs are. Look at all those happy dancing people on TV! Most doctors don't pay attention to that hype, but its all part of the sales pitch.