Why did Ya'akov see fit to relate everything that the angel told him to Rachel and Le'ah?
Ramban: Because he felt that this would convince them to accompany him when he left Charan. 1
As a matter of fact, the two current Pesukim are referring to two different dreams: Pasuk 12 goes back to when Ya'akov was still tending to Lavan's sheep (during the course of the last six years), whereas Pasuk 13 is referring to the end of that period. And on the very next day, he left Charan.