Development doesn't typically fall into any of these :).

With HA you get an SLA, a custom URL (auth.example.com instead of example.fusionauth.io) and an architecture capable of handling more users.

You might want a custom URL to test your DNS/cookie config. Or you might want to load test (please don't load test with a basic cloud deployment, it won't tell you anything about production performanc).

If you want to derisk this early, in both these cases, we recommend standing up a temporary HA instance, applying your configuration, testing, and then tearing down the HA instance.