Agronomist Luis Mendez, who works out of Sola de Vega, explains that when a stone mill is used to crush roast agaves, the crushing is uneven, leaves larger pieces of the solids, and preserves all the liquids.
Yeasts like nooks and crannies to inhabit, and they like the sugars in the liquids, so the fermentation is longer and more complete, yielding richer and more complex distillations. He says that pulping by hand with mallets in a canoa is even better.
This in comparison to mechanical shredders, what most industrial tequila producers use.
Shredding loses a lot of liquid, so the shredder is adding water, diluting what ends up in the still. Tequila producers also remove the agave solids before distilling. These are two of the reasons that good artisan mezcal is richer and more complex than 99% of present-day tequilas.