[roymurdock]

While there is agreement that inflation is costly and should therefore be minimized, for a number of reasons policymakers nevertheless aim for an inflation rate above zero. First, available measures of inflation are imperfect and tend to overstate “true” inflation. Second, a little inflation may make it easier for firms to reduce real wages—without cutting nominal wages—when necessary to maintain employment in an economic downturn. Third, a negative inflation rate—deflation— could be even more costly than a similar rate of inflation, suggesting that a low rate of inflation might be desirable to insure against falling prices. Finally, at very low levels of inflation, nominal short-term interest rates may be very close to zero, limiting a central bank’s ability to ease policy in response to economic weakness. Because nominal rates cannot fall below zero, policymakers cannot cut short-term interest rates any further once they have lowered these rates to zero. - George Kahn, VP Federal Reserve Bank of Kansas

Basically, it's because people want to see their wages go up, and a central bank loses the power to set short term interest rates when inflation is 0%, as we've witnessed in the US over the past 6 years.