Hell is the absence of God.

to reach the god tiers is to become a "fully-realised" X of Y; it's Sburb's idea of reaching adulthood by growing into the strict role it has in mind for you, so you can be a good, well-behaved parent to the universe it expects you to create.

but the role Sburb sets out for you isn't always the healthiest direction for you to grow in as a person. being a Thief of Light constantly hungering for more relevance and more fortune was holding Vriska back as a person. so the Hell Tiers were a way for her to change the trajectory of her life, into a form that better suits her as a well-adjusted adult and which can help change the course of the story as a whole.

that's my take :)