Depends on your application's risk profile and your organization's risk tolerance.

If there were no risk ie. your code will work 100% of the time addressing the business need perfectly first try, why bother writing tests?

Tests are one way to mitigate risk: the risk of pushing out bugs, of breaking existing code, of breaking teams who depend on your code, of making your code use insane amounts of cpu/memory/disk space/bandwidth, of pushing out stuff that only works for one specific time zone, locale, or culture, etc.

For hackathons, prototypes, and low-criticality systems, not very much. It may not be worth testing at all in some scenarios.

For latency/speed-sensitive systems, it may be worth spending extra time to automate those types of tests. Otherwise, it may be reasonable to skip.

For systems that are safety/financially-critical, it may be worth going beyond testing into the realm of formal methods/modeling.

Remember, tests are there to stop you from shooting yourself in the foot. If you don't care or you're shooting air, skipping tests won't give you a scare. But if they may cry, or someone could die, you'd be a fool not to even try.