When you need to run more than 1-2 applications, especially in HA, and you're cost-conscious so just throwing tons of machines in autoscaling groups doesn't work for you.

Said applications don't have to be applications that you're writing as part of your startup. It can be ElasticSearch cluster, Redis, tooling to run some kind of ETL that happens to be able to reuse your k8s cluster for task execution, CI/CD, etc. etc.