Pharmaceutical companies are in the business of making money on drugs. When they "discover" a new drug (discover is used loosely here, but that's another conversation), they have a monopoly on that drug due to patent. So once approved by the FDA, they have so many years to make back their R&D monies and fill their coffers before other companies start marketing generics.

So once the patent has expired, drug profits go down. Next step is to come up with a similar drug to the original, maybe with one little molecule changed, and they come out with a new drug that is patented and the time clock starts again. Now, they need to get people off the original drug for which the market is now flooded with generics. So they advertise this new drug on TV and tell people to ask their doctors. Doctors, instead of just telling patients that the original drug does just as well and is cheaper, will prescribe the new drug in order to 1. keep the patient happy and 2. keep their pharmaceutical reps happy.

Pharmaceutical companies keep records (legally obtained) on how many prescriptions of their drugs doctors write. The doctors who write scrips on the drugs they want, get benefits from the drug companies.

Now, I may not be up to date on the latest changes in the laws so those benefits may have been further reduced (the government started restricting this shit when it became apparent how much crap was going on). But there is still some going on. I can assure you Big Pharma will find a way.

So there is the reason for the advertising.

Case in point. Mucinex is an expectorant and thins mucus secretions. It used to be prescription only but there were lots of generics out there (guaifenesin is the generic name and is put into a lot of cough syrups). So what Big Pharma did was get it approved as an OTC drug (over the counter) as a time release, changing the formula enough to get it patented. Only recently have OTC generics become available. Prescription guaifenesin is still available, I think, but rarely prescribed since it is available OTC and therefore, insurance companies won't reimburse for it.

The drugs and Big Pharma handling of them can get a bit complex.