As far as I can tell it doesn't do anything, and the best thing to do is to add all measures as a list in one column, but then there would be a "Measures List" instead of grid.... so what's the "grid" part do?
The main reason I can imagine is that you can put a measure below a column to which it refers somehow. This is obviously the case for the standard case of a measure being the sum of a specific column. But other than that, I agree there is no real use.