Because that is how you get the dominant seventh chord. The key of C major is C D E F G A B C. If you build a triad with a added third over G (dominant), you get G B D F. The spelling of a dominant seventh chord, 1, 3, 5, b7. It's the only chord in that key that can be a natural dominant seventh chord, and because of this it strengthens the sense of key by it's relationship to the tonic. G7-Cmaj produces a stronger sense of key than Gmaj-Cmaj. Why? Because G7-Cmaj only exists within one key, C major. However Gmaj-Cmaj exists in other keys, ie I-IV in G major. So basically you could say that by having the dominant 7th chord, your securing the sense of key.