As you surely noticed, for last 10 years the browser is actively trying to replace your desktop OS, so that the browser would become the platform, instead of Windows / macOS / Linux / anything else. Imagine the desktop becoming ChromeOS on any computer, not just Chromebooks.
For this, the browser must make all the traditional OS interfaces available. It has interfaces for accelerated 2D and 3D graphics, audio, USB, process management (workers), networking (including peer-to-peer), and a number of persistence mechanisms. But before it takes over completely, it has to interoperate with the host OS a bit, too, by exposing the filesystem.
(A decade more, and maybe the vision of Inferno [1] will sort of kind of be implemented.)
[1]: https://en.wikipedia.org/wiki/Inferno_(operating_system)
For this, the browser must make all the traditional OS interfaces available. It has interfaces for accelerated 2D and 3D graphics, audio, USB, process management (workers), networking (including peer-to-peer), and a number of persistence mechanisms. But before it takes over completely, it has to interoperate with the host OS a bit, too, by exposing the filesystem.
(A decade more, and maybe the vision of Inferno [1] will sort of kind of be implemented.)
[1]: https://en.wikipedia.org/wiki/Inferno_(operating_system)