Why Should Central Banks Buy Private Securities as Part of Monetary Policy?

Infographic Instant with Bryane Michael

Central banks conduct monetary policy mostly through government securities markets (ie they buy and sell government bills, bonds, etc.) -- when not directly tampering with interest rates. Why should governments buy companies' securities directly - rather than support banks to do this? For more, see: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3023795