You use milk because it gives the sauce a creamier taste and it can give it a different texture.
You do not really need to add it if you think that it will be too rich and add too many calories to your meal, but this is what is called for in the authentic Italian recipe. This recipe was recorded in 1982.
Milk or cream is often used to make this sauce thick and hardy so that it sticks really nicely with any type of pasta you use. Also, this sauce is used to make lasagna bolognese because is so rich in flavour.