Because it's pure, you can know what comes out at the other end when you put something in. (That sounds more disturbing than it's meant to be). You can build pi

Because it's pure, you can know what comes out at the other end when you put something in. (That sounds more disturbing than it's meant to be). You can build pieces that you can reuse within many different structures, like different types of pipelines and filters. All without having to worry about leaks or the wrong stuff ending up in the wrong place.