The cellar master has to make sure the standards are up to snuff every step of the way
From the grapes to the eaux-de-vie to the aging and blending, the cellar master has to keep track of the cognac at each stage in the process. It begins when "tasting" the eaux-de-vie, done only with the nose because they're around 70% ABV. The cellar master will even pay extra for eaux-de-vie he or she thinks are of particularly excellent quality -- it's essential at this stage to decide which eaux-de-vie will become the various grades a house produces. The best might get set aside to see how they develop, which may wind up in a special release. This was the case with Rémy Martin's Louis XIII, which comes from cognac aged 100 years.
Yes, that's right: 100 years. Eaux-de-vie a cellar master selects now will make it to market long after he or she is dead. As you might expect, it's insanely difficult to become a cellar master. Rémy Martin's current cellar master, Baptiste Loiseau, is the youngest in the world at age 37... but he had to study for seven years directly under the previous cellar master before he got the gig, and that doesn't include the time he worked for Rémy Martin before that, or the advanced agronomical engineering degrees he has.