Plenty of logic is there.
Part of the purpose is to make the Doctor pay attention to the advertising that they see in a journal. [1]
How?
Patient goes in and "asks their doctor" about a drug.
Then another patient. And another.
So now when the doctor sees an ad (or the detail man comes in) the doctor pays more attention and perks up. Similar I guess to the way once you buy a particular car model (or a friend buys one) you start to notice them all over the place. Same reason Pepsi puts ads in football stadiums. Builds general awareness and adds to reinforcement of the product as being "current".
HN works in a similar way for YC by the way. Repetition and current relevance very important in advertising.
[1] Edit: And of course Physicians also watch TV and read mainstream magazines and an ad is likely to stand out and get noticed just for that. (As a tech person imagine if you saw an ad for some software or SAAS that wasn't mainstream on the nightly news. It would certainly get your attention and stand out (more than if you saw it on Techmeme)).