"A very basic creature that gets stamped on in a fight. Beetles are very weak and are regarded as fireball fodder by the stronger heroes."
— Dungeon Keeper Manual
The Beetle is the second creature to appear in the original campaign, following the Fly. Their presentation as being "tough enough to absorb a lot of damage" is tailored to this situation and is only accurate relative to the Fly; shortly after the beginning, the derisive stance taken against them by the manual becomes the reality as more powerful creatures and heroes join the action. Then, Beetles will serve no other useful purpose but to be sacrifices in the Temple.
A Beetle has no default task, with no jobs that it likes to do or is particularly good at, and nothing it hates. It's a lazy creature that will, without guidance, just idle in its Lair except to take meals and paydays. For this reason, Beetles can serve as a sort of placeholder or decoration in your dungeon. They can fill in on tasks where needed while working for spit and requiring no attention at all.