> "steamed water" that supposedly improves the taste

If that's about americano, James Hoffmann[0] covered why this might be the case - normally you use the steam wand to provide the water for the americano but that water has been sitting in a tank being heated and cooled and gradually evaporating down to concentrate all the bad stuff. If you use the wand to steam fresh water, you don't get all the crud. Seems simple and logical enough.

[0] https://www.youtube.com/watch?v=0HdzJz_evNw (skip to about 5:00 for the explanation)