User blog comment:B214/Shin DxD 1 discussion/@comment-36663391-20180920083134/@comment-3965848-20180920115928

Here's my theory, Chichigami knew Issei wouldn't be strong enough in time to face him, so she devised the 12 Heavenly Breasts to raise his chances.

We still don't know the full potential they have but it's meant to be used against the beings from the other world, so it could be the key to end this war before it even starts.