I would give four reasons:(posted 8741 days ago)--His theories dominated how people thought about business cycle issues for fifty years (and still profoundly shape how we think about them today).
--Where his attempts to shape economic policy were successful (i.e., Bretton Woods), the policies put in place were themselves extraordinarily successful: his IMF and World Bank helped sustain the most astonishing post-WWII economic boom the world ever saw.
--Where his attempts to shape policy were unsuccessful (Versailles; Britain in the 1920s; coping with the Great Depression), nearly everyone agrees in retrospect that his proposals would have provided a better road.
--He wrote extremely well: he is still well worth reading today. If you want a brief introduction to monetarism, Keynes's _Tract on Monetary Reform_ is still the best thing ever written (in my view at least). If you want to read about economic policy in the 1920s and early 1930s, Keynes's _Essays in Persuasion_ are indispensible. And his _General Theory_ is an extremely lively book--even if it is much more a prophetic vision of the economy than a work of social science...