From the "How to Use the Weapons & Artifacts Deck" section:
The Keeper can use these cards for quick referenced during a game, or hand them out to players should their investigators find weapons or artifacts while searching around, and so on.
When conjuring up a villainous non-player character in the midst of game, pull a card to ensure they are suitably armed. Likewise, if using the Curious Characters Deck, pull cards from this deck to bolster that character’s weapons or to give them an usual artifact.
The cards provide a handy guide to the item’s statistics, possible uses, and also provides some background information. Special information is also included (such an impaling damage, blast damage, etc.) on each card where appropriate.