I suspect the 4 covenants supposedly 4 of thousands in teh shadowlands were designed because they played very crucial roles.
Maldraxxus were the defends of the shadowlands
Kyrians ferried souls from life to death
Night fae - were responsible for the rebirth cycle of death to life
The Maw was obviously the prison
But what is Revendreth's crucial role? Afterlife rehab for the most sinful?