The answers are mostly correct here, but the main reason for using powdered milk as opposed to fresh milk is very few people in rural areas have refrigerators. Without a way to keep milk fresh, powdered milk is a necessity. There are still hundreds of thousands of people in Guatemala that use wood burning stoves and very little electricity in their day to day lives.
Well most of the people that have acess to cow milk will not use powdered, and the people that do not have access to cow milk like in Guatemala city use powdered to save some quetzales. U know that a galon of milk in Guate is about 5 usd compred to powdered is about a buck fifty. :(
Powdered milk is shelf stable, so if they only use a cup of milk a week for cooking or whatnot, they don't have to throw away the rest of the milk.
I don't drink milk, but I use it a lot for cooking, so I buy powdered milk.
The lady who answered first is correct but even more reason is there are many, many poor folks who don't have refrigeration so powdered, or UHT milk are about the only options they have. Canned evaporated milk is more expensive.
If they have access to fresh cow milk?
