Duty is payable on release. If you import in container-sized lots and sell in weekly lots, you are financing the government’s share of your entire consignment weeks before you have the revenue to cover it.
A bonded warehouse changes the timing. Goods sit under bond, duty falls due as they are withdrawn, and your cash flow follows your sales rather than your shipping schedule.
It is not free — there is bond licensing, storage cost and stricter record-keeping, and the paperwork on withdrawal is real work. For an importer with lumpy demand and a decent volume, it is usually still the cheaper option.