This allows you to take multiple, consecutive exposures of the same scene with the same settings and use them to average out the noise in the sky.
If you have clear skies like I did in this example, the only objects moving in the sky are the stars and the moon. This allows you to take multiple, consecutive exposures of the same scene with the same settings and use them to average out the noise in the sky. With free tools like Sequator you can align the photos around the stars, which ultimately blurs out the noise in the frame together with the foreground while the stars remain relatively sharp.