> Price stability is a good thing, and moderate deflation (which discourages productive investment) is worse than moderate inflation, so policy targeting moderate inflation is a good thing.

That is one of the reasons. But arguably, even if a central bank could be 100% sure of hitting their inflation target exactly, they might still prefer moderate inflation to absolute price stability, because (a) it encourages productive investment and (b) it gives a simple mechanism for downward adjustment of sticky nominal prices/wages.