The reason for roots?

During the recent rework on Part 2 Section 2 and 3 I spent quite a while
thinking about roots as they relate to the data model. It may be that my
imagination is not firing on all cylinders but here are the 'facts' as I see
them;

1.    A root is a node with no inbound edges

2.    There is a path from a root node to all other nodes in the graph

3.    Given 1 and 2 it *is not* possible to have more than one root

4.    Given 1 and 2 it *is* possible to have a graph with no root

So, I'm at a loss as to why I would want to label a given node as the root
of the graph unless it's to avoid the deserializer having to work it out by
inspection. Is this the reason?

All input gratefully received

Gudge