What is the relevance of checking Expense Management in 'Assign Business Functions'

I want to know that- Just enabling Expense in Offerings is sufficient for Employees to submit expense reports or we have to check the Expense Management in 'Assign Business Functions'. We have a BU where Expense management is unchecked however employees are still able to submit an expense report and get it approved. Want to know which setup is overriding 'Assign Business Functions'