Storage is a real, finite resource: tanks, silos, vaults, warehouses. Its cost is normally small and stable, so curves sit in mild contango. When storage fills, the marginal rate explodes, and when there is nowhere left to put the commodity the curve can do extraordinary things.
Goods that cannot be stored at all — electricity, some perishables — have curves driven purely by expectations, which is why power prices can spike a hundredfold and mean-revert in a day.
Example: crude tank rent of about $0.40 a barrel per month plus 5% financing on $80 is roughly $0.73 a month. A curve wider than that pays a storage trade; see full-carry.
Related: cost-of-carry, contango