If you're retail, it goes down after xmas. If you're a tax company, it goes down after may. If you sell a product, it goes down when you go long enough without releasing anything new
At night, often. For example, I have had a use case where we needed a 1000 node build farm during the day when developers were working, but only 50 at night. Machine learning jobs are another common source of workloads that need burst capacity.