I don't think that's what he meant by "I don't get it". Most of get why half weight lines exist, but why make a line full weight heavier, and label it as the lower rating? If I wanted a line at the higher weight, I'd just go out and buy one at that weight. There would be a lot more options.
There's another reason for half weight lines, btw. Some classic cane rods designed for silk line just don't have a line that balances them in the full weight ranges.