The Spectre is a stealthy and cloakable spy bot that's totally invisible to your enemy. When it self-destructs, it unleashes a 5,000 EMP damage charge that stuns units in a 360 area of effect.

Self-destruct it near an enemy Citadel to disable it with the discharged EMP.

It is also good to spot nearby enemy units so your artillery has a clear target. However, beware that it can be detected if the enemy has a Tracer within range.

Spy bots can Reclaim, which is useful for removing Dragon's Teeth or walls in its way, or slowly reclaiming enemy buildings. Beware that they must decloak to do this!