Because it's one of the key ways that physicians get new information about drugs. Not every doctor has the time or the proclivity to do research on every newly launched drug. Having a rep come in and answer questions goes a long way to getting doctors up to speed on the latest information.
This... I feel that we (US) should go back a bit on this, and restrict all advertising not in specific magazines/journals that target doctors, and restrict office visits of pharma reps to dr offices.