Two major reasons Google added this new scrollbar:

1. Stop "jumping document width madness". With the OS scrollbar (on windows at least) the document would shrink with the scrollbar width when its length starts requiring a scrollbar.

Collapsing/Expanding a thread which makes the document shorter/longer than its view space will thus shift all centered designs right/left.

2. Does not take space. The scrollbar is drawn NO TOP of page elements. It does not occupy its own dedicated space.

Even worse were always-present scrollbars even when having nothing to scroll) as misguided attempts to solve problem 1.

I for one like this solution.