You wouldn't necessarily. Main use-case I believe was FirefoxOS -- at least the first time I heard it mentioned was during an FirefoxOS talk where they discuss

You wouldn't necessarily. Main use-case I believe was FirefoxOS -- at least the first time I heard it mentioned was during an FirefoxOS talk where they discussed other hardware-y APIs and trying to get them standardised. It would also be useful for an OS that supported HTML5 native apps. Additionally having a standard API gives projects like Phonegap an API to 'target' when exposing native device APIs to JavaScript.