Commodity speculation tends to increase that commodity's price (that article explains how it works with oil, but the principles are similar). So not only is it a way to make money without actually contributing anything to the economy, it also makes life worse for the people who actually need to use that commodity since now they have to pay more for it.

Heavy speculation can also lead to price bubbles – where demand (and by extension, price) goes up almost entirely due to investor demand, and not due to people actually wanting to use that commodity. In the 17th century, for example, there was a craze for tulips that caused the price to skyrocket, simply because some people thought it would be good to speculate on.