In former churches, they were a special place, where residues, such as holy water, used up during liturgical ceremonies were sent.
Ash from burnt cotton wool was anointed with them, they were anointed or water was poured out, in which the so-called chalice linen was washed, i.e. decorative fragments of materials used by the clergy during the liturgy. All this in order to properly dispose of the remnants of what remained after the holy rite, without prejudice to what served to worship the Supreme.
Sacraria were most often located near the altar. This, found in the Opole cathedral, may therefore be another signal indicating the fact that the original church was located in this place.