What is the point of a 4-way diff? I never used a rebase workflow and I always merge. When merging you compare the common ancestor with the two different branch

What is the point of a 4-way diff? I never used a rebase workflow and I always merge. When merging you compare the common ancestor with the two different branches, and it’s the perfect use case for a three monitors setup with the three-way diff. From what I understand rebasing is pretty much the same but it rewrites history to have a single branch. Why you would need a 4-way diff in a rebase instead of a 3-way diff?