As long as Guardian Beast is untapped, non-creature artifacts you control can’t be enchanted, they’re indestructible, and other players can’t gain control of them. This effect doesn’t remove Auras already attached to those artifacts.
In this card’s heyday, your Power 9 artifacts like the Mox cycle (zero-cost artifacts that tap for one color of mana) and Black Lotus (sacrifice it, get 3 of any color mana) were safe from anything that can destroy artifacts. It also kept your opponent from stealing any of them. While there are Commander decks today that could have fun with this card, black isn’t one of the primary colors for artifacts. What mostly keeps it from seeing EDH play is its sky-high price tag ($100-150). It’s a nice card to have around, but only for collectibility.
Notably, Guardian Beast was once given out as an oversized promo card. They are quite rare. While you can’t play them in a deck, they are another piece of Magic history that the serious MTG collector would love to have.