What are NATO rounds? why?

Its just a 7.62 mm cartage that is standardized for various rifles of NATO countries. In the event the continent of Europe was to see a third "civil war" then the allied western nations could share this cartage simplifying logistics. It could also I believe be loaded in belt or box feed machine gun -- the M60 -- the biggest machine gun carried onto the battle field by NATO. It is hard hitting but heavy for the individual solder's needs. The Americans have used a smaller round since Vietnam in the M16. Other armies as well have opted for the lighter cartage in an ault rifle.