Why countries need a constitution (почему страны нуждаются в конституции)?

It doesn't if it's old enough and has a very strong tradition of the rule of law, and the powers and limits of government, and the rights and duties of the people, are adequately agreed upon by everyone. It also helps if it is somewhat monocultural, which means that people all feel part of the same nationality in the same way. But young, multicultural countries like the USA can't meet these standards. The British constitution consists of tradition and judicial precedent.