If private agents expect inflation to stand at 2% over the medium term, and if this expectation is reflected in their economic decisions, it will be that much easier for the central bank to achieve its objective. Its monetary policy is thus more efficient. This means that the desired effect can be achieved with less use of instruments.
If, by contrast, workers and trade unions expect persistently high inflation rates, they could continue to make well above-average wage demands, for example. Or enterprises could continue to raise their prices in anticipation of further increases in costs. If targets are clearly missed over a longer period of time, the risk of economic agents adjusting their expectations will grow.