To understand China's surprise currency devaluation, you need context.
China is engineering a major economic transformation — or, at least, trying. For years, it relied upon export-led growth and massive investments in housing, infrastructure (roads, rails, ports) and heavy industry (steel, glass, aluminum). This economic model now seems spent. World trade is weak. Overinvestment in housing, infrastructure and industry has left gluts. So China is switching its engine of growth to consumer spending on services and light manufacturing.
Whether this conversion succeeds or fails is a momentous story. A China that succeeds is likely to be more stable. The fate of the Communist Party also depends, to an unknowable extent, on the outcome.