Ladybird is an experimental engineering project; consider it "basic engineering" (a la basic research). Thinking in terms of "value proposition" is very limiting. Its goal is to develop an independent web stack (not a "hobby project" at all); its intent, one of several really, is to peek into and implement the internals of the modern web stack.
By doing so openly and incrementally without attachments to deadlines or an employer with specific priorities, the dev can take the time to identify inefficiencies, pain points, subtleties across the stack etc. which are then recorded in the development of the browser.
Thus, the project is not aimed to "achieve" something at some point in time; its "value proposition", if you must use this annoying term, is the development process itself.
By doing so openly and incrementally without attachments to deadlines or an employer with specific priorities, the dev can take the time to identify inefficiencies, pain points, subtleties across the stack etc. which are then recorded in the development of the browser.
Thus, the project is not aimed to "achieve" something at some point in time; its "value proposition", if you must use this annoying term, is the development process itself.