Let's say you're a person who takes a certain medication for your condition. You've taken it for years and you're doing fine with it. Now a pharma company develops another drug to treat the same condition. It claims it's better. You would have been happy with your old meds, but "the new stuff is better". So, you ask your doctor if you can have the new stuff. He probably wouldn't have switched you, but since you asked....
Also, many of the doctors get perks and kickbacks for prescribing the new stuff.
Always remember: oxycontin was supposed to be a less addicting strong opioid. They marketed it that way, even though they knew it was BS. The doctors knew it was BS and still prescribed it that way. And now look where we're at. These people don't do this for altruistic reasons. They are in it for the money. They're gonna make a buck, no matter how many people they hurt.