What are the use cases for anonymous SSL cipher suites?

That TLS includes such capability is not without good reason.

It provides for confidentiality without the need for a certificate authority - an endpoint must be configured to remember what certificates it will accept, instead of which certificate authorities it will accept. This is a completely different trust model from that generally used on the internet, and should not be used on a public website.

I would not expect this cipher to be available in any browser.

What are the use cases for anonymous cipher suites on a website?

None. This is just a severe mistake and therefore the grade is capped to F. None of the browsers offers anonymous cipher suites (at least by default) so no connection with a browser will be established this way. But it might well be that some mobile banking apps make the same mistake.