Terraforming

It depends.  Sometimes it's a lifesaver, but most often it's just a cheap way to smooth a bumpy road (lame pun intended)

It only a small part of Earth Magic, and pales compared to Giant Form, Nature's Cloak, etc...

That said, I have used terraforming to:

1. Let a city expand into swampland.

2. Allow a cornered unit to escape.

3. Create a direct route to enemy territory.

4. Smooth the road for an important stack.

5. Improve a settling spot.

6. Destroy a settling spot.

I frequently use it for exploration - either to allow my scouts to bypass horrible critters without ending a turn next to them, or to create a path through mountains in order to see what's on the other side. 

I also use it to bring my cities closer together - creating routes around troublesome forests or swamps, passes through the mountains, and even the occasional bridge across a lake.  This works well coupled with a unit that can create new roads.

I have also used it to slow invasion forces - every turn new hills rise up in front of them.

Sometimes, when you lower a hill, that act will create a 'tile yield' plot where you can create a city.

And you can raise lands in the coast to create a land bridge, or to get new tiles available to settle

I used it in my last game to make enough room for a city to build a logging camp that kept giving me an insufficient space message.  And enchanted hammers and earth elemental are both very useful.  

Check out https://forums.elementalgame.com/436647 for a fun spell/magic mod by mqpiffle that has some neat earth spells as well.

Once, while I was invading, the enemy had two cities separated by a mountain range on one side and a river on the other. I had tons of mana, so I flattened the mountain and charged through.