The Alter Rebbe told this story :
One day gehennom burnt down. The malachim were thinking of building a bigger and better one, but then the tzaddikim complained that gan Eden will be outdated while gehennom is freshly renovated.
The malachim acceded to their point and decided to refurbish gan Eden, and make the old gan Eden the new gehennom.
One way of understanding this story is as follows:
Hashem wants us to do Torah and mitzvos, but it’s hard sometimes to overcome going along with the yetzer hara. So He created gan Eden and gehennom to keep us in line.
In the generations before the Alter Rebbe there was allot of maggidim who would rouse the yidden to teshuva and becoming better people with their vivid descriptions of gehennom.
To be continued as this is long.