Node is required because that's what is used to compile the JS on the command line, it's a nodejs process. Once that's complete, any web server can serve the newly produced JS/CSS/HTML files.

JS is interpreted by the browser, but we compile locally so that the code produced implements workarounds for missing language features. The code produced is typically minified so that the payload sent to the browser is much smaller than it would otherwise be.

Node is analogous to Python, Ruby and PHP... they all can run webservices, power desktop apps and compile/build projects.