If the result of processing a POST would be equivalent to a representation of an existing resource, an origin server MAY redirect the user agent to that resource by sending a 303 (See Other) response with the existing resource's identifier in the Location field. This has the benefits of providing the user agent a resource identifier and transferring the representation via a method more amenable to shared caching, though at the cost of an extra request if the user agent does not already have the representation cached.
If I interpret the spec properly, they are offering an option to return a Location containing the url of a request that results in a resource representationand at the same time, redirecting the agent to the url. If a representation is returned immediately after the POST (common with many APIs today), it should not be considered cacheable as it was not fetched with a GET that would return cache control headers, an ETag, and the last modified date. This paragraph describes a way to accomplish both the initial POST request and returning a cacheable representation of the resource, but at the expense of another http request by the agent as it follows the location in the 303 response using a GET.