The dominant 7th creates strong tension that pulls the harmony away from home and then back again, making the resolution to the I chord feel especially clear and satisfying. As you listen, pay attention to how the V7 feels unstable and how obvious the return to the tonic sounds when it resolves. You’ll hear this cadence all over music, in endings, cadences, and key-defining moments, being able to recognize it will make identifying tonal centers much easier.