Why do currencies devalue?

in most cases, state currencies devalue because of
economic downturn inside a country. This cause balance of payments deficit and the inflation rate to grow.
This can be a sequence of different economic downturns such as war actions, GDP decreasing, commodities devaluation serving as exports basics, purchasing power falling, credit conditions tightening, political instability inside a country and etc.
Devaluation (currency devaluation) is often tied with incorrectly organized monetary policy and relating decisions of fiscal controls (Central Banking System).