Monsters
Yes, typical I know, but that's what makes it easier to digest right.
Anyway, although monsters are a part of this world due to their mutation from mana, fissures (also known as gates, portals, dungeons e.t.c from so many other novels/manhwa/ light novels you may have read) began to appear around the time mana also came to this world.
If the fissures were not cleared in due time, monsters would grow strong enough in the space created by the fissures and after feeling that they were strong enough, they would make their way out of it and begin to wreak havoc and chaos. As a result, various guilds were created to accommodate rankers who were periodically and frequently tasked with clearing out the fissures.
From it, monsters that were killed were able to provide raw materials which are then used to make various types of equipment, weapons, e.t.c This in turn provides a rich and stable source of income to whomever is ready to risk their lives and clear out as many fissures as they can.
The first volume of my novel may not really touch on it all that much because it's really just a setup for the world itself but it definitely will be crucial to some aspects of my story and will even be used quite a lot in the second volume (I pray to God that I'm able to motivate myself that far) and even the later ones.