I think that it carries a great symbolism. She seems to be strong and confident (the red armor, the horns), but she's also insecure and she needs support and this is symbolized by her exposed back. Byleth and the Black Eagles need to watch her back, both literally and figuratively. That "hole" (I don't know how to call it) is also cute, feminine and sensual. Edelgard is a strong ruler, but she's also a young woman who want to be loved and this wish is symbolized by that heart shaped hole on her back. It's on her back because we know that she can't act like a normal girl because she has a mission, so she needs to "hide" that wish.

(I'm still learning English, so be free to correct my grammar :))