Suggest you go on internet, my friend - Wikipedia.
But for ICEs
Extends engine life removes carbon build-up in engine Extends muffler, oil and gasket life Produces no emissions produces clean water at exhaust reduces engine heat reduce sound (quieter) improves torque & power in conjunction with HHO generator, EFIEs or MAP/MAF greatly improves economy engine can double its life expectancy (if used when vehicle is newish.