Initially, aiming at 2% allowed some core economies to lower relative prices in a fairly painless way, as other “catching up” economies had higher inflation rates and were pulling up the area average. Today, the positions of those groups have reversed. But the rationale remains exactly the same.
The second reason why a 2% objective helps absorb shocks is that it supports the implementation of monetary policy in adverse conditions. A small positive buffer creates more scope to support the economy through cutting nominal interest rates and reduces the likelihood of running up against the effective lower bound.