You’ll often hear a recording artist, or their hype person, say this at a show.
When people make noise, it’s easier for the artist to believe that everyone there is interested in what they’re doing, and to bring 100% to the performance. This creates a self-fulfilling prophecy; the audience, in turn, is energized by the artist.
Even if they don’t know the artist’s music—and they probably won’t!—an audience will usually be willing to clap, cheer, or make a bit of noise. They’re there to have a good time.
If you’re an artist, knowing how, when, and what to ask will get you really close to what you want.