This song was everywhere that summer and as a song it’s nothing special, but after seeing the movie you subconsciously fix it permanently in your mind with this unforgettable and revolutionary scene.
The music builds to a crescendo while he sprints up the many flights of stairs, all the way to the top, and the camera spins around him to show the skyline as he suddenly raises his fists triumphantly … chills, every time.
Scenes like this became common, but this was the first.
Stereogum has a good summary of how unique and revolutionary it was.
With Rocky, director John G. Avildsen more or less invented the training montage, a convention that would become a staple of American movies for about the next 20 years. (We still get training montages, but now they’ve become a self-conscious, parodic trope. Often, they still use the same music as the Rocky movies.) In Rocky, Rocky Balboa has randomly been booked in a fight with the champion of the world. He’s only in there because he has a good nickname, because he comes from the right city, because he’s white, and because he can’t possibly win. Rocky knows he can’t possibly win. But thanks to his hardbitten, motivational trainer, that starts to change. He starts to believe. And then the montage hits. ... As we watch Rocky, we can imagine “Gonna Fly Now” as the music in his head, as his own monosyllabic inner monologue. Rocky loses the big fight in the end, but he wins respect and becomes a contender. He achieves his destiny. A downbeat bummer of a story becomes a triumph for the ages, a harbinger of an era when movie ticket buyers demanded uplifting spectacle. “Gonna Fly Now” has everything to do with it.
Exactly right.