Sainte Chapelle was originally built to house the relics of the Passion of the Christ acquired by the monarch during the 13th century. These include a piece of the True Cross and the Holy Crown, which nearly disappeared in the early 19th century.
Starting in 1790, Sainte Chapelle was restricted to members of the clergy and was emptied of its contents. It progressively became used for the storage of archives from the neighboring Palais de Justice - Paris’s Court House - until a restoration campaign was started as a result of pressure from the public.
Today, it is no longer possible to admire the relics for which Sainte-Chapelle was built, as they were either lost or displaced during the French Revolution. However, visiting the chapel is a worthy experience in itself!