Well that's what the seed companies are advertising, but if you have a polytunnel or a mild climate and can cover your crop with soil, polythene or fleece when it gets cold, then I don't see why you can't plant them in the ground as normal. The advantage of sacks is that you can move them under cover if frost threatens (or snow as last year) but potatoes in the ground last fine as long as they are well earthed up or covered with a thick layer of straw. But if you have suitable containers it might be worth trying both methods for comparison.