Millions of people spending billions of dollars and talking about trillions of ideas 24 hours a day. You can reach a few of those people through word of mouth. Mass media includes a lot more options than you're probably thinking of right now. The key to using mega-media is to know exactly which members of the public each one is aimed at reaching. Television has a very different audience than any particular radio station.
Magazines generally touch a national or regional readership.
Billboards are good at reinforcing name recognition or a message you have already established on one of the other media. But you don't want to do just billboards by themselves. Direct mail, or junk mail as most of us call it, is very good at picking out certain kinds of people who form your top prospects. Direct mail pays best when you use it in a big way, and that can cost some bucks. Advertising people who represent the mega-media would like you to think their advertising rates are very high.