As you know, we've been living in a RankBrain world for some time in Google. Recently there has been some coverage on it around Google not fully understanding it, as well as RankBrain not impacting crawling or replacing links.
So Gary Illyes posted on Twitter this morning another nugget saying "RankBrain lets us understand queries better." He added that there is "no affect on crawling nor indexing or replace anything in ranking."
Lemme try one last time: Rankbrain lets us understand queries better. No affect on crawling nor indexing or replace anything in ranking— Gary Illyes (@methode) March 18, 2016
So Rand Fishkin asked him to clarify, here is that tweet:
@randfish to the best of my knowledge that's the case, yes. (I am on the search quality team, but not rankbrain, so I don't know EVERYTHING)— Gary Illyes (@methode) March 18, 2016
So it seems RankBrain doesn't add any new ranking signals but based on the query, it may weigh the existing signals differently. That is if Gary Illyes understands how RankBrain works, which he said he might not.
In short, it seems the query triggers all of this and RankBrain's job is to figure out what to do based on the query.
Forum discussion at Twitter.