The idea with the named delegate would be if you need some way to:

    delegate Task<string> GetUserEmail(int userId);

This provides more guidance than taking in a:

    Func<int, Task<string>> getUserEmail

If you can annotate implementations of the delegate the tooling support becomes even nicer. Not all Funcs with the same shape have the same semantics, in my ideal C#-like language.

Edit: I completely forgot the main reason which is if using a DI container it can inject the named delegate for you correctly in the constructor. Versus only being able to register a single func shape per container.