At the beginning of Diana's story, we learn all about the God Killer sword, a weapon left by Zeus to be used against the resurrection of Ares. By the end of the story, we learn that the God Killer was nothing of the sort, and that the ultimate god-killing weapon was actually Diana herself, since she was actually a demigod, and the daughter of Zeus. But Diana's mother, Hippolyta, pointedly refuses to reveal Diana's true nature. She says she does this to protect her from being discovered by Ares, but that explanation doesn't quite wash, since Ares pretty much seems to know her deal throughout the whole movie. So why keep the truth from her?
The reason, it seems, is that Diana needs to experience the difficulties of war, loss, and love so she can tap into her true powers and defeat Ares. We see in the movie that simply punching the War God isn't enough to make much of a dent. But when Diana realizes her love for Steve, and the value of his sacrifice, she finally believes in the true potential of mankind.
Love, not war, is man's real purpose on Earth.
Well, that, or because discovering she's a demigod makes for a more dramatic reveal near the end of the movie. Either way.