Manual testing has a real place, especially for judging whether something actually feels right to use, but it has a specific, well-documented weakness: it only tests what the person testing happens to think of, on the day they happen to test it. A regression in a code path nobody manually clicked that week ships silently, and stays silent until a user finds it.
Automated tests don't have that blind spot in the same way, because a well-written suite runs the same exact checks on every single change, not just the ones a person remembers to retest. That's the actual value: not that automated tests are smarter than a human tester, but that they're exhaustive and consistent in a way manual testing structurally can't be at any real scale.
The category that catches the most real bugs before launch is the boring one: unit tests on business logic (pricing calculations, permission checks, data transformations) and integration tests on the paths that touch a database or an external API. These catch the class of bug that's invisible in a quick manual click-through: an edge case in a calculation, a permission check that only fails for a specific user role, a race condition in a background job.
Load testing catches an entirely different category that manual testing can't touch at all: what happens when fifty people use the app at once instead of one. Connection pool exhaustion, N+1 queries that were invisible with ten rows of test data, memory leaks that only show up after sustained traffic. None of that shows up in a demo.
The realistic goal isn't 100% coverage; that number gets chased for its own sake more than it should. It's coverage on the paths where a bug is expensive: payments, auth, data that can't be easily corrected after the fact. Everything else can lean more on manual review without much real risk.
