Really loose recollection and paraphrasing;
Trace contaminations in alcohol could lead heavy drinkers to pass out in a coma-like state for potentially days at a time. Given the frequency, holding a wake to be absolutely sure it wasn't one of these poisonings became a useful custom.
And then of course, changes and regulations mean the contaminations are weeded out of the process, but the ritual itself persists because of human habit.
Still waiting for the Harpy to be thrown out on her ass.