引用官方的原文来解释一下这个问题

What is the Extensions Filter for?

Some MyFaces components do more than include some HTML in the pages. They may need additional support scripts, style. sheets, images, ...
Those resources are included in the MyFaces' jar file and the Extensions Filter add the code and URL needed to provide those resources to the generated HTML.

Some other components, like the file upload component (t:inputFileUpload) need to parse Multipart requests.
The extensions filter handles this as well.

Why is this useful?

This design has several benefits :

1. It provides a clean separation between MyFaces' components and your webapp.
2. You don't have to include additional MyFaces' components related code or resources in your pages or webapp.
3. It provides great flexibility to the MyFaces' team upgrade the components, while keeping transparent backward compatibility.
4. It makes it possible for complex components to use many support resources without any burden on the page developer.
5. It loads only the resources really useful for the rendered components.
6. It handles the MyFaces' resources caching.

How does it work?

When a component needs a resource, is calls one of the org.apache.myfaces.component.html.util.AddResource methods (for example AddResource.addJavaScriptToHeader(InputHtmlRenderer.class, "sarissa.js", context); ).
This method add an attribute to the request so that the filter knows that is must include the given javascript, stylesheet or resource link to the page.
The code for this resource is automatically included in the generated HTML.
The URL for an embedded resource is constructed and always begins by /myfaces/ so that it can be intercepted by the filter when the client need to load the resource.
When the clients fetches the resource, the filter decodes the URL, and serves the proper resource that is embedded in MyFaces' jar.

How do I configure it?

In your web.xml, map this filter to the path used for the JSF pages (most likely *.jsf) AND to the /faces/* path as in the following example :

Under what circumstances am I *required* to use the extensions filter?

If you just use standard JSF component, but don't use any MyFaces' extended component (beginning with t:), then you don't need the Extensions Filter.
However, if you use some of the MyFaces' extended components like t:inputFileUpload, t:inputHTtml, t:inputCalendar, ... then you most likely need to have this filter configured in your webapp.

Does this impact performance?

The filter hasn't any significant impact the response time.
However, as the filter has to cache the whole response in memory before writing it out to the client, it slightly increases the memory usage.