First, one should note that language planning is not universal. Some countries have strongly centralized and official forms of language planning, as in the French Académie française, while many others do no form of language planning at all. There are several reasons why language planning is sometimes important to some countries, having to do with specific cultural circumstances.
Language planning is an important element of cultural preservation for groups who have been oppressed or see themselves as a cultural minority. For example, in Canada, language planning addresses potential inequities between anglophone and francophone Canada by legislating equal treatment of two national languages to ensure that the French cultural heritage of Canada is preserved and retains parity with English.
Language planning is common in postcolonial countries that are attempting to reassert their own languages and traditions after periods of colonial oppression when government and even elite education was conducted in the languages of the colonial powers. Ireland, Scotland, and Wales have successfully revived their own Celtic linguistic traditions and cultural heritage after centuries of English dominance. Similarly, Europeans tried to suppress Native American and First Nations languages in North America, and the revivals of these languages have been important in the revival of indigenous culture.
India is an interesting case, with over 100 languages. India has retained English as the official language of government as a compromise so that no one Indian language ends up being favored. Although many Hindu Nationalists would like Hindi to be the official language, speakers of other Indian languages would find that oppressive, giving excessive power to one religious and ethnic group among many.
Language (i.e., code) planning is the deliberate effort to control, expand, and utilize language. Cooper's definition of language planning is:
language planning refers to deliberate efforts to influence the behavior of others with respect to the acquisition, structure, or functional allocation of their codes
Language planning is important to a country for several reasons. The first is that planning is important to insure that a language corpus can function in contemporary society in terms of terminology, or vocabulary, to meet present needs, e.g., technological, or scientific needs. Thus, Cooper describes corpus planning as intervention to make sure a language has the needed terminology to function in necessary capacities.
The second reason is to establish the status of a language within a country and in relation to other countries in the world, for instance, it establishes whether a country will have one national language or two and which those two might be. As an example, South Africa effected status planning by determining that both Afrikaans and English would be official languages.
The third reason language planning is important to a country is that language planning determines how language or languages will be acquired, or taught, in the national educational systems. Acquisition planning comprises acquisition of national, second, and foreign languages.