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Catacombs are human-made subterranean passageways for religious practice. Any chamber used as a burial place is a catacomb, although the word is most commonly associated with the Roman Empire.
Simply so, why did they build the catacombs?
To make room for more burials, the long-dead were exhumed and their bones packed into the roofs and walls of "charnier" galleries built inside the cemetery walls.
Also to know, what is an ossuary and what is its purpose?
An ossuary is a chest, box, building, well, or site made to serve as the final resting place of human skeletal remains. They are frequently used where burial space is scarce. A body is first buried in a temporary grave, then after some years the skeletal remains are removed and placed in an ossuary.
A small portion of the Catacombs is open to the public or tourists. Entrance to the official Catacombs is restricted, though, and only consists of a tiny part of an extensive network of underground tunnels which spans more than 300km (about 186 miles) in length.