They know how the business works and who to talk to. A few hours in a studio with an experienced engineer can produce wonders. If your band has been performing and practicing together regularly, you should be able to go from one song to the next, just as if it were a gig. House Of The Rising Sun by the Animals was recorded in one take because they had toured with the song for so long, it was second nature. Once you have a quality recording of your band, send it to record companies and radio stations. You can also produce CDs and sell them at your gigs.

Summary:
Get an agent. Once you have more spending money, invest in studio time.