It's about being able to use more pixels to render things like curves and serifs in fonts mainly - things that subpixel hinting attempted to address on lower DP

It's about being able to use more pixels to render things like curves and serifs in fonts mainly - things that subpixel hinting attempted to address on lower DPI displays can now be done much better and without the blur tradeoff by using more pixels. At the very same size, fonts look noticeably better because more pixels can go into those tiny rounded edges.