The answer is simple, balance!
When dipping that thief into the barrel a million thoughts go through your mind, tannins, colour, acid, flavour but most importantly the balance of all these elements.
Firstly lets take a look at why wine is barrel aged. Barrel aging is the step that takes place between fermentation and bottling. Wine is ultimately barrel aged for longevity, flavour development and maturation.
When wine is in a barrel there are many factors that can influence the wine positively as well as negatively therefore the wine needs to be tasted on a regular basis. Tasting the wine regularly and monitoring the development thereof helps manage the micro oxidation, metabolic reactions and proper integration of wine and oak.
Ultimately a barrel tasting is important to ensure the wine is softening over time due to the micro oxidation, the tannins are stabilized, the oak flavour is transmitted and the wine is in balance.