Question

why does a milkman adds a very small amount of baking soda to fresh milk?

Answer 1

By adding a very small amount of baking soda to fresh milk, the milkman keeps the milk unspoiled for little more time than usual time.
As the pH value increases the milk turns to slightly alkaline, 

Answer 2

Milkman adding small amount of baking soda, the milkman shifts the pH of the fresh milk from 6 to slightly alkaline so that he can keep it for longer time as the milk in alkaline condition, does not set curd easily.