[rwh86]

Hard cheeses contain almost no lactose; it's consumed by bacteria during the maturation process.

[njarboe]

A way to store a surplus food that contains high protean and fat is another huge benefit of dairy. Hard cheeses can be stored for years in the right conditions and these conditions are achievable for ancient people (caves).

[eru]

And if done right, the cheese becomes tastier as it ages.

Not as important today, but more important back then: aging your cheese also concentrates the salt. Salty foods are tasty.

[njarboe]

Good point. Back then salt was valuable in places just short distances from the ocean. Humans need salt, especially when doing hard physical labor, sweating all day.