This is because most central banks these days focus their monetary policy attention on some concept of core or underlying inflation, and ignore the transitory impact on prices of exchange rate movements, movements in international oil prices, changes in indirect taxes, and so on.
And focusing on core or underlying inflation means to a very large extent trying to assess whether there are unused resources in the economy. In other words, keeping core inflation under control once price stability has been achieved is to a very large extent about keeping the estimated output gap as close to zero as possible.