Copper tools have a longer life span; their edges stay sharper for longer; they do not rust; and they come out of the ground cleaner because the copper is less resistant to friction. Of most interest to gardeners is the deterrent effect of using copper on slugs and snails. This is related with the electrical properties of the copper and how that interacts with the soil.
Jane Cobbald – How I learned to value slugs and snails
Metalle in der Medizin