Do threads save memory?

Question

Node.js

Hello!
I watched a screencast on nodejs. There, the author clearly states the following position:

It is better to use streams as they save memory.

I agree with him. If we are talking about writing to the response stream (res), then everything is clear. We are gradually sending the answer in parts, we sent several parts - the buffer was cleared. And so in a circle.
When reading, everything is also clear. We read the file piece by piece.
But where does the memory saving happen in this code:

// ...
      var body = '';

      req
        .on('readable', function() {
          body += req.read();

          if (body.length > 1e4) {
            res.statusCode = 413;
            res.end("Your message is too big for my little chat");
          }
        })
// ...

We just take and shove all the contents into one variable. So why is there a stream?
Thanks in advance to everyone who helps!