So I'm a bit confused about what exactly happened at the Battle of Jakku.

From what i understand, part of the Contingency Plan was for the Empire to retreat to Jakku and regroup there to destroy both fleets or something like that (?), and then the New Republic came and defeated them.

So what exactly changed story-wise? During the Battle of Endor, the Death Star II was destroyed, both Palpatine and Vader died, and the empire was left scattered. Did they just regroup, got defeated again and were left scattered again?

Just trying to understand the point of it, wouldn’t the story just have followed the same if there was no Battle of Jakku?