I don't know about the case with Carvel style boat-building, but the slot in the head of the mallet helps the hammer create a "clinking" sound when in use... which lead to Lapstrake boat construction also becoming known as "clinker" built. Otherwise, the sound would be more of a "clunk". . . and nobody wants a clunker.

Okay, that's all just made up stuff but that's the story I will tell from here on out.