Based off of the blog post previous to this one about the implementation of SVGs site-wide, I'm confused why they'd risk using SVGs over just CSS alone or CSS+P

Based off of the blog post previous to this one about the implementation of SVGs site-wide, I'm confused why they'd risk using SVGs over just CSS alone or CSS+PNGs or even matte'd GIFs? Maybe I'm behind the times, but that seems like a nightmare to maintain comparatively.