What is the significance of monopoly power?

Monopoly power has the power to do good and evil. Good e.g. through economies of scale and an efficient allocation of resources; evil in that it can, in the absence of competition, restrict ouput and therefore customer choice, as well as charge a higher price than in a contestable market.
Other things to consider include whether it minimises costs of production, how it erects barriers to entry to prevent other firms joining the industry, and what you think government action should be (if any).