I get Kaz’s point here – it’s not just about tagging enemies (though Corruption could be used for that). There are occasionally certain targets that are weak but need to be killed almost immediately, and ideally at range. The eggs that spawn webs in certain delves are an example, as was mentioned.
If you play Destro, you’d have Shadowburn or Conflag. Affliction can use Agony/Wither/Corruption. But when you play Demo, the best option is either Corruption or a /petattack macro, which feels janky even when it does the job.
On top of it, most classes, including melee, have something that can handle those targets in an instant, so Demo feels like an anomaly.
You’re conflating two requests here. One is that the spell exists in our spell book. The other is that it is designed to be part of the intended main damage rotation. I think most people want to keep it for utility, and not necessarily make it a button you push when doing your normal rotation.
If you are hung up on why would Warlocks have a “useless” damage ability from a tuning perspective, or are afraid of it being somehow shoe-horned into the rotation due to some theory crafting, I’d say it’s somewhat equivalent to Heroic Throw for Warriors. They don’t need to use it, but it exists to do damage in cases where instant ranged damage would be helpful.