The CMB radiation was emitted 13.7 billion years ago, only a few hundred thousand years after The Big Bang. It was even before stars or galaxies existed. By studying the detailed physical properties of radiation, scientists can learn about conditions in the universe on very large scales, since radiation we see today has traveled over such a considerable distance.
The supposition that the universe began in a microscopic super dense, hot state, which expanded into the cosmos we know today is called The Big Bang Theory. The early state implies that the universe should be full of radiation. When the universe expanded, the radiation cooled and is now blackbody radiation of three degrees. When the background concerning radiation was discovered, it supplied a fair amount of evidence that there is some truth behind the theory.