The distinguishing feature of all mammals is the female's ability to secrete milk for the nourishment of her young. So for most mammalian species, Lactation Matters simply because the species would not survive without it.
Actually, milk is much more more than just food, and lactation means much more than just survival:
So lactation matters because breastfeeding provides the very best start in life, but that is by no means the end of the story. For centuries, the milk of our domesticated dairy species has provided mankind with an invaluable source of high quality fresh and conserved food. As our human population increases and becomes more affluent, lactation will matter more and more as our dairy industries help society to achieve global food security.