At the end of Captain America: Civil War, Zemo reveals that
he trapped Bucky to make the Avengers fight each others.
But around the middle of the film, he
in order to learn what happened on 16 December 1991.
It doesn't seem to be an essential part of the plan, and everything has already happened when all is revealed. Actually to me it looks more like a matter of luck if he has something he could use it
to make Iron Man fight Cap and Bucky
they all made a truce... Which happened because Tony knows that all is Zemo's fault.
So why does he want so much to know that?