Why use Organizational Hierarchies for Security? How does it flow through the Org. Hierarchy?

I am trying to find a reason on why we should use Organizational Hierarchy when it comes to Security. How is it helpful? Given that now we can assign different roles to users in different companies through the 'Assign Organization' feature, the org hierarchy seems redundant for Security.

For example:

TestUser can be assigned an Accountant role in Company ABC. But we can also add another role (say Buyer) in DEF company.

This shows that company ABC and DEF don't really have to be 'under an umbrella' since Security allows individual company assignment.

Is there anything that I am missing or not seeing clearly from this concept?

Any help or insight will be much appreciated.

Thanks.