The goal of most any recording artist or group is to sell its music and get touring gigs. The means to this end is to get their music on the radio. But, does a song have to make it to the Top 40 and stay there for a while in order to "make it" and make money? In most cases, yes. The difference can be selling 1,200 albums in a week versus selling 1,200 in an hour.
Moving up the charts
With costs of producing an album, promoting it, going on tour, and everything else that is involved, a musician has to sell a lot of music in order to make money. Selling more CDs means more touring opportunities, and more touring opportunities means selling more CDs and getting more airplay. It becomes a cyclical thing that feeds on itself: charts are based on airplay and records sales; record sales improve with more airplay, which keeps the song on the charts.
However, with Billboard's shift in ratios, weighting record sales lower than airplay, the key to staying on the charts is now staying on the radio.