Richard D. Wolff

A full discussion of exactly the question you pose is available in the following book: R. Wolff and S. Resnick, Contending Economic Theories: Neoclassical, Keynesian and Marxist (Cambridge: MIT Press, 2012). Briefly, Keynesian economics is a set of theories and policies designed to protect and preserve capitalism by using government interventions (chiefly monetary and fiscal policies) to offset and/or minimize those capitalist contradictions (especially its instability - business cycles) that undermine and threaten its legitimacy.