h.265 (HEVC) compresses fairly differently, the modes and fallback methods are also better the higher the detail (dimensions)... with x.264 (AVC) you get pixelation, with HEVC you get blurring... for a 1080p or 4K output, this is MUCH more pleasant to watch and allows for some shortcuts that aren't as noisy.
One of the first things I noticed when first seeing DirectTV and DVD quality, for example was the pixelation of things, usually in the background... to this day it sometimes distracts me... that alone is about enough for me.
The way regions are selected for compression is also better, but I'm not into the technical details enough to explain well... all I know is I can get about 5/3 to 5/2 more content in HEVC while at an equivalent or better quality. So yeah, it's a big deal...
