Similar to credence, or belief in Italian, in the Middle Ages credenza was the act of tasting or testing foods and wines so they could be served to a master—with assurance from a credible servant that they were safe to eat.
Later the term referred to the room for this tasting—probably between the kitchen and the dining room—and still later, to the furniture the food was placed on.
Originally waist-high for convenience, with storage cabinets to keep serving utensils and table linens, and often topped with a sheet of marble or a linen runner. Later, fine china and small treasures were protected behind glass doors of taller styles—going up as rooms were made smaller and less communal.
Since furniture has been commercially marketed, they’ve been included in dining room sets so all the pieces match—table, chairs and storage. Now synonymous with a buffet—where all food plates sit cafeteria-style on top of a sideboard, or credenza—this casual style of self-serving a large party versus formal table service for each guest, suits relaxed gatherings in modern, open-plan homes.
So just as formality has relaxed—an engraved invitation can be a simple evite or tweet—credenzas can be used for dining, media or office storage, and mixed with various woods or lacquer finishes.