What are use cases of AWS spot instances?

Spot instances are unused EC2 instances available for lesser price than on demand instances.

This helps a lot in cost saving. At particular time of day when usage in a particular region goes down (during night hours maybe) the spot price also decreases. You can schedule your heavy workflows on cluster of spot nodes at that time and this will not only make instances available at lesser price but now you can use lot many more instances and speed up your processing.