The Imperial Drones are akin to genetic debt collectors of troll society, and facilitate the reproduction of the Troll race. They travel around Alternia collecting genetic material that has been combined by Trolls paired both as Kismesisses and Matesprits. They bring this genetic material to the Mother Grub, who combines it all in an incestuous slurry and lays hundreds of eggs at once - genetic material from stronger bonds of either nature see more representation in the resulting brood, while weaker relationships result in more recessive genetic traits. We should all take a moment to lament how pedestrian the human reproductive system is, and further lament that the phrase is not a feature of common parlance in human civilization.
Failure to contribute to each filial pail when prompted results in death. Trolls unable to find a mate have nothing to contribute to Troll society and are therefore removed from the gene pool. This is referred to as "culling", although failure to contribute to the filial pails is not the only reason a troll can be culled. Consequently, the Imperial Drones may be related to the Culling Drones mentioned, potentially even being one and the same.
If Trolls are anything like social insects like ants or bees (like they have been implied to be quite often), then it is probably that the Drones are in fact born of the unfertilized eggs of the Mother Grub.
They were created by the Condesce after she took over Earth and initially were part of her scheme to force the Alternian reproduction system upon humanity. After humans go extinct they serve as a sort of "police" of her twisted regime, patrolling the world in large numbers and carrying out her orders. Unlike their Alternian counterparts they are robotic instead of organic. Like all other Crocker Corp tech they are bright red.