I'm learning Node.js and often enough in the tutorial exercises the assignment is to take some kind of input through process.stdin , modify or filter it and pass it on to process.stdout, meanwhile as a clue there is example code that uses http.createServer( function(req, res) { req.pipe( function of some sort... res.end(), and other times it can be something like this process.stdin.pipe( function "something perhaps involving the through module"...).pipe(process.stdout) .
The difference is that the former (processor.stdin/out) is writing a command line program. The latter (http.createServer) is creating a tiny web-server, where "input" (the page request) and "output" (the resulting pages) will basically be managed by the browser via HTTP. Tutorials which do not demonstrate anything web-specific may use the command line because it is simpler.