A preemptible VM is an instance that you can create and run at a much lower price than normal instances. However,

Cloud Computing Services provide information technology (IT) as a service over the Internet or dedicated network, with delivery on demand, and payment based on usage. Cloud computing services range from full applications and development platforms, to servers, storage, and virtual desktops. [Source: Dell]

Engine might terminate (preempt) these instances if it requires access to those resources for other tasks. Preemptible instances are excess

Cloud Computing Services provide information technology (IT) as a service over the Internet or dedicated network, with delivery on demand, and payment based on usage. Cloud computing services range from full applications and development platforms, to servers, storage, and virtual desktops. [Source: Dell]

Engine capacity, so their availability varies with usage.