According to Wiki:
This value (between 0 and 180) controls the threshold for this tool to work on adjacent triangles. With a threshold of 0.0, it will only join adjacent triangles that form a perfect rectangle (i.e. right-angled triangles sharing their hypotenuses). Larger values are required for triangles with a shared edge that is small, relative to the size of the other edges of the triangles.
However, I don't quite follow that part (even trying my best to), and still puzzling on how it works exactly.
Can anyone explain more about it, for easier understanding?
P.S.: I know what it is for, just can hardly understand how it works from Wiki.