One big reason people use multiple regions or clouds: higher resource availability.

Allocating scarce resources can be very hard on the cloud. These are things like high-end GPUs (both on-demand and spot; the latter being much harder to get) and also beefy CPU-based instances. We've seen 10s of hours of waiting times or longer.

The natural solution is to have the flexibility to allocate resources in multiple regions (and ultimately, clouds) to increase the total pool size.