Do you mean why couldn't you get the same concurrency using multi-threading? You could but that doesn't fit in all environments like node.js. Check out how ko

Do you mean why couldn't you get the same concurrency using multi-threading? You could but that doesn't fit in all environments like node.js. Check out how koa.js can leverage similar concepts to reduce nested callbacks. Also beyond the concurrency the lazy loading aspect can have implications for functional programming techniques.