Marketing consists of the introduction of a product or service to individuals or groups that may or may not be willing to purchase this product or service. On a national level, marketing would be important as a means to maintain a positive national image on a globalized scale. For example, if a given country wished to promote their country as a tourist destination, it would be in their best interest to release media that highlighted the positive aspects of their society. This would include information about their economy, relevant local attractions, and even the weather. The same could be said about individuals or organizations. An individual can market him/herself by making a resume that highlights their various positive qualities and accomplishments. This would be necessary in order to secure most jobs. Likewise, an organization may market itself in order to motivate consumers to buy a certain product. This would include highlighting information about the product that would make the product more attractive to consumers. In all cases, marketing is necessary in order to make a product or person seem attractive and relevant, which is important in any type of collaborative interaction.
The importance of marketing is the same for all of these types of entities. They must all market themselves so as to project the best possible image to the world. By doing so, they will be able to get others to think well of them.
The type of entity does not matter -- the goal of marketing is still the same. A corporation, for example, wants to sell more of its products. To do so, it needs people to think well of it. Therefore it needs to engage in marketing. A country wants to have a good image in the rest of the world for economic purposes and/or for purposes of political power and prestige. To get such an image, it needs people to think well of it. Therefore, it needs to engage in marketing.
Marketing is always about making the general public perceive an entity in a positive light. All entities want to be perceived in this way and therefore they (when they can) engage in marketing.