Why do I need outbound rule in reverse proxy

Hi,

I'm learning about the ARR reverse proxy. According to my understanding, the IIS responsible for replay the request to the backend. So I think the inbound rule in enough to return the correct content. However, in the reverse proxy with URL rewrite module tutorial, it display the steps to set up outbound rule. Could any one explain why I need to use outbound rule ? Besides, is it possible to use outbound rule to rewrite the content between A tag?

Thanks in advance.