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Why this project?

It's either an Integrated Authoring Environment as its targeting tech-savvy people (not necessarily developers, but also people who work with them).
You could compare it to a good old top-down CMS (as opposed to headless): it gives you the head (Astro) and the spine (content structure + management) back. Authoring and previewing living content in the same place has its advantages, this is the case for LAMP stacks which has handled this for ages.

Astro + Content gives this power back, without all the complexities, oddities and maintenance burden of traditional CMSs (with all due respect).
Opting for this symbiose between code and content has took a big leap, thanks to headless, "JAMStack" frameworks which popularize Markdown and MDX as a first class citizen, with a whole set of eco-systems around these.
For example, Astro Content makes validating Markdown front matter part of its core. Though still far from perfect, every step like this one is done to catch up a bit with high standards developer experience (e.g. TypeScript).

Tooling has still a lot of room to grow for MD(X), and things are a bit scattered in these areas. In fact, some of them are in their infancy.
That's what motivated Astro Content: easing environment setup and usage. The more people use these tools, the more incentive these will get to be improved.