First off, 2-row has a better flavor profile to use as a base grain. It's got a high diastatic power so it's capable of converting itself as well as a good amount of specialty grains which have been toasted or roasted and have no enzymes of their own. However, the famous american lager makes use of a very high percentage of adjuncts like corn and rice. When using a lot of rice or corn (say 40% of your mash), the 2-row has insufficient diastatic power to convert itself and all the adjuncts. That's where 6-row steps in. Although it doesn't taste as good as two row, it has super high diastatic power and can convert itself as well as all the adjuncts. As far as I know, that's pretty much the only reason to use 6-row over 2-row. For it's conversion capabilities.