Automated scanners find known patterns. A penetration tester, often called a pentester, chains small issues together the way a real attacker would: a guessable ID here, a weak permission check there, and suddenly they are reading another customer's data. That human creativity is what the test buys you.
It is done with written permission and an agreed scope, covering which systems, which dates, and which techniques are allowed. You receive a report with findings ranked by severity, proof of how each was exploited, and fix advice. After you fix them, a retest confirms the holes are closed.
For an AI-built app, a test is most useful before launch, before you handle payments or health data, and when a customer or investor asks for one. Fix the obvious issues first with scanners and reviews, so the tester's time goes to deeper problems. Choose a provider with verifiable references, keep the scope specific, and never let anyone test a system you do not own or control.