A sense of (generally well-regarded) design direction made by professional designers. Familiarity to Android users. Not reinventing the wheel and doing a worse job at it.
> And why a whole custom distribution, instead of a desktop environment?
It's easier to make an integrated desktop if it isn't distribution agnostic. This way you can easily depend on distribution-specific stuff like which version of Wayland it has, depend on systemd, PulseAudio, which package manager it uses, what font settings it has, which GPU drivers it uses. If you make this a desktop environment instead of distribution, then you need to make it work with a far more diverse set of variables, which I think is undesirable when you're starting out (or even in general).
Your first reason is a bit condescending. KDE Plasma, Unity, Elementary, etc. all have clear design guidelines, and are all made by professional designers. They are not any less professional because you disagree with their choices / reasoning, and it's not clear that what makes Material Design work on mobile will translate well to the desktop at all.
I agree, and the reason is mainly lack of good design tools on Linux. It wouldn't take much though, perhaps there will be a Qt based design tool in the near future which will have a Linux port, and we'll see more designers (especially web designers) moving over.
If there was Photoshop or Sketch for Linux, and a lot of people could migrate. And no, GIMP or Inkscape don't cut it
> What does Material even offer to a Desktop OS?
A sense of (generally well-regarded) design direction made by professional designers. Familiarity to Android users. Not reinventing the wheel and doing a worse job at it.
> And why a whole custom distribution, instead of a desktop environment?
It's easier to make an integrated desktop if it isn't distribution agnostic. This way you can easily depend on distribution-specific stuff like which version of Wayland it has, depend on systemd, PulseAudio, which package manager it uses, what font settings it has, which GPU drivers it uses. If you make this a desktop environment instead of distribution, then you need to make it work with a far more diverse set of variables, which I think is undesirable when you're starting out (or even in general).