Hierarchy allows you to build the structure, which allows you to see / locate where is my "selected" position within the tree structure.
For example, if you build an "Employee" hierarchy based on employee vs manager, then you can find who this employee reports to. At the same time, let's say you're trying to find out how many hours you work this week, then you can see the hours comparison amongst:
1. other employees under the same manager; OR
2, other employees under different manager, but still on the same level as you are.
HierarchyBelongsTo allows you to build a structure which allow you to see the total hours of all employees under your manager. Please check out this link, as it helps how I understand it: