And to further what Alina says it's used for Blossom end rot as that's a calcium deficiency. Actually most of what I've read says you can use powdered or proper milk, but full fat not non fat. Classed as an environmentally safe fungicide. Well you live and learn!
BER is related to calcium in that it's a side effect of a plant not being able to distribute calcium within its internal system. There can be tons of calcium available to the plant but the plant can't use it. So milk won't help against BER. So what causes the plant not to be able to distributre calcium? No one knows for sure, but plant stress seems to be a major factor. Strong buffeting winds, temperature extremes and irregular watering are only some of the possible causes.
There is also the puzzling fact that some tomato types - the plum varieties, for example - are more prone to BER than others. No one knows why.
Milk has no value as a tomato fertiliser but it's been claimed over the years that it has anti-fungal properties. The evidence is largely anecdotal, backed by the suggestion that the milk creates a pH level that fungal spores don't like.