It's not necessary, but efficiency makes prospering easier by lowering the required effort. If efficiency weren't important, nobody would every bother to optimize (aka. plan) anything.

Also note that planning and central planning aren't synonymous. A central planning process makes more efficient use of planning resources, but that doesn't help if the plans it produces are themselves inefficient because they're based on five-year-old statistics someone faked for their personal gain. So far, decentralized planning seems to be more robust overall, and the additional decision-making overhead is small compared to the cost of actually moving things around.

In that sense, it's not even the case that food distribution is currently unplanned; a great deal of effort goes into determining what quantities of various foods should be transported to warehouses all over the world in order to meet future demand. The problem is just that the planners are optimizing their own profit, and keeping the largest possible number of people well-fed isn't profit-maximizing currently.