If Cubase came from using the legacy GDI video API (before Direct2D / Windows 7), which it most certainly did, it’s either still using GDI today or, most likely transitioned some or all of it to Direct2D calls which are highly compatible and interoperable with GDI.
If so, then a video card’s GPU will assist with Direct2D calls when drawing all primitives (lines, rectangles, etc.).
I suspect Cubase 8 sees some small UI performance gains when paired with a reasonably modern card it’s happy with dirver-wise.
This is for Windows. I think OS X and its OpenGL is hard to avoid these days; stuff is gonna get accelerated without too much work from the coder. Less of an issue with Mac, anyway, as the platform video choices are narrower and mostly meet a minimum spec, with driver issues more stabilized.