As a software engineer I often interact with documentation for software projects across the Internet, in particular on GitHub which provides methods for documenting your project. Anecdotally, it seems like more projects are using GitHub's built-in documentation nowadays but many projects, especially popular ones, often have their own dedicated website, which effectively just offer documentation and download links, the same as GitHub.
While on one hand I think having a separate website adds a level of legitimacy to the project or the ability to fully customize the styling, but the overhead in creating such a website seems cost-prohibitive, at least at face value.
I feel like I've seen this in other contexts as well, where documentation that could have just been built into a standard system is instead partition off to a separate website and linked to from the expected location.
Are there other perceived advantages to this approach from a communication perspective? Is there maybe an advantage to having your documentation in a space that is free of other context? Are there any disadvantages to this approach?