To appreciate why market prices are essential to human well-being, consider what a fix we would be in without them. Suppose you were the commissar of railroads in the old Soviet Union. Markets and prices have been banished. You and your comrades. Passionate communists all. Now, directly plan how to use available resources.
You want a railroad from city A to city B, but between the cities is a mountain range. Suppose somehow you know that the railroad once built. Will serve the nation equally well. Whether it goes through the mountains or around. If you build through the mountains, you'll use much less steel for the tracks.
Because that route is shorter. But you'll use a great deal of engineering to design the trestles and tunnels needed to cross the rough terrain. That matters because engineering is also needed to design irrigation systems, mines, harbor installations and other structures. And you don't want to tie up engineering on your railroad if it would be more valuable designing those other structures instead.
You can save engineering for other projects. If you build around the mountains on level ground. But that way you'll use much more steel rail to go the longer distance and steel is also needed for other purposes. For vehicles, girders, ships, pots and pans and thousands of other things.
Which route should you choose for the good of the nation? To answer, you would need to determine which bundle of resources is less urgently needed for other purposes. The large amount of engineering and small amount of steel for the route through the mountains, where the small amount of engineering and large amount of steel for the roundabout route.
But how could you find out the urgency of need for engineering and steel in other uses?