What are the benefits from this?

I'm not trying to be dismissive, it's just my feeling from working on search.marginalia.nu is that nearly every aspect of search benefits from locality, not only is the full crawl-set instrumental in determining both domain rankings and relevance signals on a term-level such as anchor tag keywords; but the way an inverted index is typically set up is extremely disk cache friendly where the access pattern for checking the first document warms up the cache for the other queries, but that discount obviously only exists when it's the same cache.