In most cases, the multi-company needs to be set up for subsidiary companies. What they need is a consolidation of the balance sheet at the end of the financial year, even though there is no parent-child relationship between companies.
In some cases, finance offices want to see ongoing transactions for the entire subsidiary without switching between companies. Odoo allows you to switch between companies, and you can get the information for that company.
The ongoing transactions can be available to the finance officer. Just by defining the parent-child relationship between companies, the officer can track and manage the approval and rejection of the invoices or any other documents:
Look at the preceding screenshot. It shows you how to configure Your Company as a child company by selecting My Company in Parent Company field.