The simple answer is:
It's a chelating agent... a synthetic amino acid, if I'm not mistaken.
It binds with metals (minerals) and keeps them from interfering with the cleaning process.
It's also used in chelation therapy for arteriosclerosis. It's typically injected via IV, but is also included in oral chelation formulas.
Think of it as a water softener of sorts, when used in a cleaning chemical...