The main purpose being: you’re using npm anyway, so using it directly to run scripts removes a dependency (i.e. gulp/grunt/whatever) and brings you a bit closer to the tools you are using directly. I could see this example `package.json` file from Graham Smith being highly referenced as people try this out for themselves.
Remember Damon Bauer covered this here on CSS-Tricks last year, with a whole bunch of examples of what is possible.
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Using npm as a Build Tool is a post from CSS-Tricks