We did not plan our language, but the British had the most power. They also expanded more. So when it came time to make a new country, most of the people were British or American born and spoke English. The other immigrants had to learn to speak it too.
In the days of the French and English colonies, the native tongue of the conqueror was established as an official language for the government. And, to this day, these languages remain the official ones for some African countries, for example, since the native dialects often have no written language. This lingua franca of French or English enables the residents of these countries to all communicate as well as to communicate with others internationally as well. However, many of these same peoples maintain their native tongues.
I am not sure that it is essential in every country, but it can be essential in multiethnic countries. The reason for that is that a multiethnic country needs to have one lingua franca. Therefore, language planning must be done so as to establish and teach that language. At the same time, planning must be done so as not to destroy the culture and language of people who speak the language(s) that do not become the lingua franca.