"There was no reason at all it to exist. Nothing useful in the economy. There was no basic trade going on. Only spice trade."
There is a huge amount of trade in the Dune universe. Caladan exports Pundi rice, Hagal jewels, Theilaxu was the source of a lot of medical supplies, the list goes on. Yes, Melange is the most valuable substance in the universe, but certainly not the only one.