In war, a Heracross horn can serve as a catapult or a battering ram; indeed, both weapons were invented to free up powerful Heracross for hand-to-hand combat. In the popular sport of Goal Roll, it has made them great scorers, a fact true of many other sports throughout the ages. And in pokemon battles, it allows them to use Megahorn, an astonishingly powerful bug attack which knocks out countless pokemon in a single strike, especially psychics, grass types, and the forces of darkness.
It is for the latter reason that when a Heracross dies its horn is typically removed and used as a totem to protect the holder from evil. The horns themselves are fairly large, but are lightweight enough to allow Heracross to swing them around and humans to lift them without difficulty. Their carrying case must be clear, however: a hidden horn is no protection to anyone. Their power is partially explained by the fact that dark-type pokemon fear not Heracross but their horns, for a hornless Heracross is nothing but easy prey, yet most accounts suggest it defends against bad luck and villainous humans just as well as from dark-type pokemon, and an enormous body of evidence suggests this is no mere superstition.
Although it protects the holder from evil, however, it often simply trades one type of foe for another. To large flying pokemon such as Pidgeotto and Fearow, a Heracross horn is not frightening, but delicious, and these birds are often found mobbing those who rely on them for luck. It is not wise to seek a Heracross horn for protection unless one is prepared to defend it from flying pokemon, but many Bug Catchers who outlive their Heracross do so to honor their memories and maintain a piece of their power all the same.