Why would having the enzyme lactase as an adult be an evolutionary advantage?

It would be an advantage because the adult will have more variety of food that he/she can digest.

Lactase is an enzyme that digest lactose which was mostly found in milk. Lactase is important in children since for a few years of life the child got breastmilk from the mother. But after that, the source of lactase will be reduced. The case will be different if you live in a place where cow are domesticated and there are some farm that produces milk. In that case, having lactose as an adult will allow you to get energy from drinking milk and increase your food option.