Deflation is a more troublesome phenomenon than inflation in the sense that central banks' ability to respond to the former is limited.
Thus, it is desirable to have a buffer for monetary policy; that is, to secure a certain "margin" of positive inflation and positive interest rates in advance, so that central banks can take appropriate responses through monetary easing if the economy heads toward deflation. What these two reasons imply is that we should aim at a slightly positive inflation rate.