Topography allows you to explore custom terrains to make new worlds that much more interesting. You can make the nether a more terrifying place by generating it with cell noise. This new Nether has vast land masses jutting in random directions and lava pouring from the ceiling. You can even remove the ceiling of the Nether entirely.
Or you can generate a Sky Island realm where anything darker than light level 7 will deal damage.
Or you can go a bit more whimsical by creating a Sky Island world where all the islands are made entirely with a random assortment of blocks. You may come across an island made of diamond blocks or one made of spider webs!