This is due to the fact that any company’s maximum cash flow does not always come at the end of any financial year, but can come evenly anytime in a year.
In the absence of a mid-year norm, we would use discount period numbers of one for the first year, two for the second, three for the third, and so on.
Instead, we'd use 0.5 for the first year, 1.5 for the second, 2.5 for the third, and so on with the mid-year convention.