Hi, Welcome to JustAnswer! My name is Eric and I will endeavor to answer your question.
The long term results of effective product placement are just now coming in. It is effective... ESPECILLY long term. When Dunkin Donuts placed their mugs in High School Reunion back in 1997 what did they get? So far 15 years of advertising, every time someone rents the movie from then until forever.
Dunkin Donuts, FORD and Coke were some of the first to really get it. Did you realize that Hawaii 5-0 had product placement agreements in 1968? They all drove Fords. Why? Because The Crown Victoria was to cop car of the day in 1968? Nope, because Ford paid them to do it.
I'm not even going to go into the subliminal aspects of the effectiveness which some research suggests might be even more effective than what's they put "in your face". When I say subliminal I am not referring to subliminal cuts that are placed in the film for one frame, I am talking more about the ones we don't notice that are maybe more distant but where the popular logo is seen and the placement that is only there for a second or two where our conscience may not have time to say, "Oh Look, a bag of Fritos", but still has proven to make some of us get off the couch, go to the pantry and grab the bag of Fritos!
To get right to your actually question, I think the truest reasons it has become so critical is because:
A. It works and that has now been proven over the long haul.
B. It is forever as long as re-runs are still on TV and Movies can still be rented.