Anodes are installed for sacrificial protection / cathodic protection. Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte, causing one metal (the less noble or more reactive metal) to corrode whilat the other metal (the more noble or less reactive metal) is unchaged. Anodes, are metals like zinc or aluminum, are installed on the boat's hull, so the anode "sacrifices" itself rather than let the fittings corrode.