It's meant to point out the irony - Fred Astaire had the greater "reputation" as a dancer, perhaps, and Ginger Rogers was seen as something of a sidekick.
But since dancing might actually be harder for a woman - due to the high heels and such - it's meant to point out that she also deserves credit for their performances.
Sort of like saying "for every successful man, there's a woman behind him". Don't underestimate the Ed Nortons and Barney Rubbles of the world - being a second banana is also hard work.