I think the GitHub page is probably a bit clearer than our website right now:
https://github.com/plasmicapp/plasmic
Basically, it's a visual page builder that plugs into your own Next.js/Remix/React/etc. codebase. Your marketing/content/design team can then build and publish pages without filing tickets on eng. This (content management) is the main use case.
The most interesting aspect is that editors can drag and drop React components from your own codebase as building blocks. This is very powerful, esp. with e.g. data fetching React components.
Why open source? Because we love the tool. Plasmic has been in production deployment at companies big and small. But we think it can grow far beyond ourselves as an open source project. We’d love to see a broader community take it in unexplored directions. Community users have even started using it to build emails with React.Email and mobile screens in React Native.
Would love to hear folks' questions and feedback!