What's different abput it then normal milk.

it has had the fat on the top skimmed off and replaced by water xx

low fat or slim milk is 2% or less. Regular milk is 3.5% which is very little difference in fat content but very much in flavor content.
The fat in milk helps it to have a better flavor than the low fat or slim milk has. Fats also tend to help one feel fuller sooner, so one doesn't over eat as much.
Any milk is good for weight loss as long as one doesn't over indulge in it. A serving is 8 ounces or one cup measurement.

Slim milk? or Skim milk.?
Obviously because there is less fat.
Regular milk has a higher amount of fat. Read the ingredients, it should be mentioned.