The benefits of these two systems are of very different sorts. A market economy's benefit is freedom and efficiency. A command economy's main benefit is the ability to pursue specific, socially desirable goals.
Market economies allow people more freedom. The people get to decide what to do with their property. These economies are also more efficient in that they can quickly respond to the demands of consumers. By contrast, command economies are best at pushing societies towards goals that the government thinks are good for the society. As an example, let us say that global warming is truly a threat that should be dealt with. A command economy could simply require that cars get more miles per gallon and that consumers use less electricity. It could push society towards a socially desirable goal much more quickly than a market economy would go.