The foundation bylaws have this to say about the purpose of the foundation:
The purpose of the foundation is to maintain and encourage development and usage of the free software Syncthing and its required infrastructure. The foundation can provide grants on its own initiative, or by application according to the rules put forth by the board.
This is quite general, and in practice it means the foundation pays for the infrastructure required to run Syncthing (like the syncthing.net website, discovery servers, build servers, and so on), but is open to providing grants for specific projects that further the cause.
The underlying REST API has some similar issues, in that it also has grown from something small and simple to something large and complex without the benefit of planning.
We would now like to fix these issues with a redesign of both, in concert.