For your first question, IMO the purported advantage is mainly convention at scale. There's nothing inherently wrong with raw CSS in style tags or other authoring models (well, except CSS-in-JS at runtime...). Tailwind is one simple authoring model that works at scale without fuss and bikeshedding. Wrote up my experience with the advantages and disadvantages on this though a bit ago to be able to point to[1].

For the second question, depends on your definition of "meaningful" I guess. I doubt the original goal was to make money. There's OSS less prolific than Tailwind that makes money. Is it unreasonable for those projects to seek ways to compensate their projects?

[1] https://wlls.dev/blog/on-tailwind