The same reason the Himalayas are the border between China and India, the swiss and Italians border in the alps, the US and mexico border along the Rio Grande, and the Arab countries have foggy borders. It's the reason you see ethnic groups bounded by geographic artifacts. You want a natural barrier as a defense. You expand or contract to an easier to defend position. You'll note nations with artificial borders such as latitude lines are at peace with one another, and ones that aren't, such as several African nations who's borders were decided by the French and English are in constant strife and flux.
I think his (perfectly reasonable) point is that, in the long run, a natural border might make it easier to establish and maintain a legal border, and will likely favour internal cohesion by forcing connections to go certain ways.
Obviously it is not a prerequisite for statehood, which is a complex construct that goes beyond territorial claims (since Woodrow Wilson at the very least, in legal terms).
