Chart of accounts - parents and subaccounts

Hi

This seems a very basic question but we are just setting up NS and I cannot find anything anywhere that explains what a subaccount actually is and how they should or could be used (in relation to chart of accounts). For instance, if I create:

Telephone Costs (being the parent)
  Land Lines
  Mobiles
  Broadband
with the last 3 as subaccounts - why is that any better than creating just 3 accounts:
  Land Lines
  Mobiles
  Broadband

I can't find anything that makes the parent a non- posting account? that the subaccounts would roll up into in reporting. So I seem to be completely missing the point. I'd ignore it apart from having read the white paper, it recommends using parent/child accounts where possible rather than a linear COA.