Agree to disagree. Banking, telecommunications, insurance, automotive, retail are all industries where I have seen wild load fluctuations. The only applications where I have seen constant load are simulations: weather, oil&gas, scientific. That’s where it makes sense to deploy your own hardware. For all else, server less or elastic provisioning makes economic sense.
Edit to answer the last question: to test variable loads, in the last one. Imagine a hurricane comes around and they have to recalculate a bunch of risk components. But can be as simple as running CI/CD tests.