While the short scene establishes that there is at least one other Amazon living among Earth's mortals, it's first and foremost a way to honor Carter's legacy as the original live-action Wonder Woman. In an interview with Glamour, Jenkins revealed she actually wanted to include the actress in the first film, but she knew it would be too jarring when she was trying to establish Gal Gadot as the new Wonder Woman.
“It was very hard in the first one because I didn't feel like it would be right to make her Diana's mother or something like that because we all associate her so much with being Wonder Woman,” Jenkins explained. “Gal hadn't stepped into that fully yet, and you can't make a Superman movie where Christopher Reeve plays somebody else because we're all just gonna say, that's f***ing Superman! Superman's right there!”
However, the sequel presented the perfect opportunity to cast Carter as a legendary Amazonian who paved the way for the women of Themyscira to live in peace. “What I loved about this way of doing it is that I feel like we're really truly honoring who she is,” the director told Glamour. “Who is it that inspired the legacy of the greatest hero of all time: Lynda! So it was like a perfect thing to put it both in the story but actually have it stand for exactly what she means to us.”
So while Wonder Woman 1984‘s post-credits sequence may not set up a future movie (although Asteria could certainly appear again), it does celebrate Carter's role in making Wonder Woman one of the most recognizable and influential heroes in DC or Marvel history.
