A lawn isn't a natural phenomenon as you so easily put it :p Different species of grass have different environmental requirements. A standard lawn as most people have is usually made up of different species of grass and they will grow.
However, you'll soon notice after one or two seasons if you left it unattended, weeds and WILD grasses will take over because they're more adapted to the conditions at that point (light, drought, temperature etc) and eventually you'll end up with a meadow looking Serengeti so to speak :p
Grass has because of it's shallow root system a much harder time in dry weather. If it goes unwatered long enough it'll die and its place will be taken by a plant that's much less demanding in that regard. So watering a lawn is not only to keep it green, but indirectly also to keep out weeds. A strong, healthy, well watered lawn is a weed suppressing lawn.
