This answer is different from 20 years ago. Upgrades used to be for quality, or weight (my folder I upgraded from aluminum arms with swaged-on steel ring, slightly wobbly, new crank and rings run perfectly true).

But the newer design 2-piece hollow-spindle cranks (my generic is similar to "Hollowtech 2") are a significant advance:
- Lighter, and stiffer spindle (probably not perceived)
- External bottom bracket bearings, which means more bearing balls, bearings closer to crank arms so spindle more in shear and less bending, ability to easily adjust out bearing slack (unlike with cartridge BBs) which greatly prolongs bearing life due to maintaining proper preload.
- Crank can be removed with only an allen wrench, no crank puller needed.

Only downside is, drive side spider is fixed, you cannot space it out further, whatever spindle length it comes with is it, unlike ability to get cartridge BBs in many lengths. But usually this is not necessary; Whether 1X, 2X, or 3X, the chainline, i.e., the middle of the chainrings is usually the middle of the cassette, if designed for your rear dropout spacing (O.L.D.).