While the pieces of design systems make sense in and of themselves (you can argue for a clearer, easier to read font), I think a lot of bullshit gets dragged in along with people wanting to embrace "design systems" for the sake of doing it, or because it looks good on a resume "X designer built a design system at Y company". If I were to think even more cynically than usual, you can think of design systems as bundles of components.
> What is the purpose of Spectrum without an implementation like this? > Why would Spectrum be a public published thing, considering that only Adobe makes Adobe products?
Cynically -- marketing for adobe, and some easy press, internally maybe a deliverable for the design/frontend teams that was on the roadmap for this quarter. Less cynically -- other companies can choose to use Adobe's design system instead of creating their own, also everyone benefits if Adobe has done stuff that goes above and beyond the usual bundle-of-components (ex. their React Aria[0] effort).