We must know much about many things in general. Our talent is in being sensitive to how people live and respond to the made environment. We use this ability to design things to enhance people's lives. To be a furniture designer requires artistic ability, knowledge of engineering and manufacturing methods, understanding of historic styles, and for free lance designers, business ability. Actually one can be a furniture designer without a degree.
Most furniture designers do have degrees.
Surveying the education credentials of members of the ASFD reveals members with degrees in various majors from about 50 different universities and colleges from around the world. Interior Design is closely related. Many states require licensing to be a fully-accredited Interior Designer. One of the main showplaces for this is the Contemporary Furniture Shows in New York and other metropolitan cities. Speaking strictly of furniture design, Kendall College of Art and Design in Grand Rapids is outstanding.