I learned this trick from my great aunt, who used to put dishes of stale beer out to attract and kill various pests, including slugs. I knew it worked it, but I don’t think I ever quite knew why. According to a short article in the September issue of Mental Floss, Why Do Slugs Love Beer?, the answer is that the “sweet smell of yeast attracts slugs to beer like moths to a flame.” Quoting Ian Bedford, head of the John Innes Centre’s Entomology Facility in Great Britain, “A lot of slug species feed on decaying plant material,” adding that “beer resembles overripe fruits, which burst with naturally fermenting yeasts that slugs can’t resist.”
Works for snails (slugs in a shell) as well; dumping salt on slugs is another tried & true method (causes a “meltdown”, which is more cruel than letting them die happy).