What’s the need of Acceptance testing when all other levels of testing have been done over an application?

Prima facie, we might understand that after unit testing, component testing, integration testing and system testing is done, execution of acceptance testing is of no use apart from just an addition burden, but we need to understand acceptance testing is mandatory because:

a) Both the developers code and test the application based on the requirement documents and their own understanding, which may not be what the client/product owner expected.

b) Also, while the project is in development phase there are various changes in requirements that may not have been communicated effectively within the team and might have resulted in wrong business flow.

Because of the above few scenarios this becomes important that acceptance testing is performed by the client/product owner to ensure that the developed application is in accordance to specification as well as business needs.