2. Competition If you know better alternatives might exist, yes, you can search for them. But how do you search for better deals, services, or products for every little thing in your life? You don’t. Nobody has the time (or cognitive bandwidth) to proactively research every option. When done right, advertising helps level the playing field by putting alternatives in front of customers. And in doing so, it also pushes businesses to keep their offerings competitive.
I'm not convinced #2 is true — all ads imply the thing advertised is the best deal (where "best" is somewhere on cheap-quality spectrum), and the same limits to cognitive bandwidth mean we can't easily guess whatever points were missing from, at best, a 30-second highlights reel.