Put simply: Donnie is the new Jesus.
He "operates in God's channel" with knowledge of the future and Frank is there to keep him on-track, to make sure that real Frank gets shot in the face. After that, Donnie is on his own to make his choice.
The entire film is about things ****ing up because of him and him sacrificing himself to fix it.
Also, the annoying teacher woman is right all the way through. Donnie *****es about the emotion-line of love and hate, he tries to argue that there are other things, but in the end the entire film boils down to love and hate.
Oh and there actually is an answer and a real point to it, because Jake Gyllehnalanlenyall said he knew what it was in some interview.