I think a prophecy child is expected out of the union between a CD and their worst enemy, either a general D or a very specific clan, like the Davy.
Maybe this child is needed for the war announced by the Harley text, and who gets control of him has a higher chance of surviving the conflict.
If would explain why the CDs went about abducting commoner women, aside from the psychopathy of inflicting pain and misery onto another.
I doubt this is to do with needing shanks for war or to win the war
To me, this has more to do with fate.
We know very well that current era was prophesied.
Shanks inherited Roger's will and his fate revolves around two key elements.
1) he's the one who inspired Luffy and he's the one through which Luffy indirectly end up getting Nika df.
2) He's the one who is destined to fight BB and lose. Now, this could mean two things
A) This fight will end up as a catalyst to set Luffy vs Bb because we all know how much Luffy cares about Shanks.
B) shanks perhaps belonged to a family which in the past had feuds with Davy clan.
So, shanks is not only the inspiration for Luffy to sail on the journey and become Nika.... he's also the one who will spin the wheel of destiny by making sure Teach and Luffy ends up crossing each other - two massive clans of ancient kingdom fighting in the present era - and, shanks will be one of the major reason.
There could be more - like doing something to WG but let's see