Fair question! I want to say really, the goal is just to find a way to make an actually useful library documentation tool like this that people enjoy using. Myself, Beyang (CTO), and Quinn (CEO) have all wanted something like this for a while and think there's potential for something cool here.
It'll require _a lot_ of iteration to make this work really well, though, and make it something that everyone feels good about using *for their projects*. Don't want there to be barriers to using it, if it was enterprise/paid it'd be tough to do that.
There will be features/functionality doctree _can_ gain if you connect it to a Sourcegraph instance, but largely because they'd be impossible to do otherwise:
* Usage examples - we need statistical analysis of a large corpus of open source code to find good real-world usage examples, so we'll leverage Sourcegraph for that (it already has that data.)
* Respecting repository permissions, OAuth integration, etc.; very important in enterprise environments, super complex/annoying to do. Sourcegraph already has all this data about your github/gitlab/bitbucket repos, user accounts, etc. and so maybe you can one-click connect doctree to a Sourcegraph instance to gain this functionality if you're some large enterprise that needs it.
I think there are some great synergistic ways doctree will work with Sourcegraph if you use that (or are OK with it contacting Sourcegraph.com for public code, but very important to make that respectful / opt in.)
I want to be clear, though, doctree is 100% open source, it'll be a proper OSS project - just want to make a useful tool for everyone first and foremost.
I interpreted your question as "How does it make sense for Sourcegraph to build this as an open source thing, and not as a paid product?" which I thought was a super reasonable question.
I think "Why is this open source?", though, is what got you down-voted because it implies it should be closed source, when folks obviously prefer open source.
