Metal is needed where the load is high but the section must stay slim. A wooden staircase to the first floor needs a stringer as thick as your palm; a steel one manages with an eight-millimetre plate and looks lighter.
The second use is frames for furniture and appliances. A wall-hung unit two and a half metres long will sag over time on furniture panels alone; on a steel frame it will not.
The third is railings and balustrades. What matters here is not the metal itself but the spacing: less than twelve centimetres between the verticals, or a child will climb through.
Metal is priced separately from furniture because it is made separately: welding, dressing the seams, primer and powder coating — a different workshop and a different schedule. Metal usually arrives on site before the furniture, because the furniture is fixed to it.