Mead yeast nutrients are essential for the proper fermentation of mead. Honey alone does not provide enough nutritious materials for the yeast to consume to properly ferment without the addition of nutrients or other fruit additives. Unlike grapes, which are a nutrient-rich medium for yeast, a must of honey, water, and mead yeast will often stop fermentation prematurely due to stress on the yeast. Ceasing the fermentation early leads to a lower alcohol content and off flavors.
Many times, we recommend a process called staggered nutrient additions. By spreading out the number of occasions that you add mead yeast nutrient to your must, the more likely your yeast will maintain a healthy fermentation for the entire duration you want it in the primary fermenter. The process of degassing, adding nutrients, and staggering nutrient additions is recommended but not essential.