Unity is not a "file format". It's a tool and development environment that reads and writes and edits lots of different "file formats", for which programmers can make runtime and editor components, that artist and level designers who don't know how to program can plug together and configure. And it can create applications for a wide range of different platforms, not just asm.js/WebGL.
It's nice that you're happy coding things up in JavaScript and WebGL just for the web browser, but Unity has a much wider scope than that.
Eventually somebody will write something like Unity that runs entirely in the web browser, and figure out how to make it target all the different platforms that Unity does (once Apple has their brain tumor removed and supports WebGL in their iOS browser for use in anything other than advertisements). But right now there is nothing like it for web development, and it brings a lot to the table that many people really need.
It's nice that you're happy coding things up in JavaScript and WebGL just for the web browser, but Unity has a much wider scope than that.
Eventually somebody will write something like Unity that runs entirely in the web browser, and figure out how to make it target all the different platforms that Unity does (once Apple has their brain tumor removed and supports WebGL in their iOS browser for use in anything other than advertisements). But right now there is nothing like it for web development, and it brings a lot to the table that many people really need.