The account tree starts at the top level:
Corresponding to each account node is the net value of that account. I.e. the total values of all accounts with the node as it's parent. Click on the arrow to the left of the account to unravel the child accounts:
There's an account tree whenever it makes sense, i.e. when analyzing net worth, understanding profit and loss, checking cash positions, etc.
The summarizing nature of parent accounts are useful to get a good view of spending categories, asset positions, ratios of income sources, etc.