The fact that Offred, whether she survived or not, was able to leave a record of her time in the Republic of Gilead. Even if know-it-all professors are skeptical of her accounts, she was able to reclaim something that Gilead took from women: a voice.
The Epilogue reveals a few things: that the Gilead regime had ended and a new world was brought up, one where women regained their rights e.g. Professor Maryann Crescent Moon.
In the movie: Kate/Offred ends up killing the Commander with Mayday later rescuing her. She was true to her word that no one would take her baby or control her, and the shots of the battle between Gilead and the Rebels is nothing short of amazing.