According to Witch Cassandra, although the Wizards sealed the body of Everett into the dimensional space, there were too many Wraiths. The dimensional rift alone could not contain so much Wraith energy.
Once the compressed Wraith energy broke through the dimensional space, the entire Western Continent would become a living hell.
Therefore, every once in a while, the Wizards of the Western Continent would organize a group of Wizards to enter the dimensional space to clean up the Wraiths inside.
"...Cleaning up the Wraiths is the responsibility of every Wizard of the West Coast. No matter what identity or where you come from, as long as you are selected, you must unconditionally shoulder this responsibility."
After listening to Cassandra's description, Heath could not help but ask, "Why did they choose me?"