The text says:

>When processing a "POST" (create), the full URL is treated as the id of the resource on the source, and is ignored; the server generates an id for the resource. For updates, the server performs a mapping between the fullUrl specified and the local URL the server knows that instance as, if possible. If the server does not have a mapping for the fullUrl, the server ignores the base URL and attempts an update assuming the base is the same as the server base. This allows the same transaction bundle to be sent to multiple systems without changing the fullUrls for each target.

I think this is a left-over from when entry.request.url did not yet exist - since this has taken over most of the functionality described above. What remains is that the fullUrl is used to be able to reference between resources in the bundle - even when these entries will get be a assigned an as-yet-unknown id.