By now, most of what I write about is stuff you can genuinely check yourself, with no security background: two test accounts, a private browser window, a search through your own code for exposed keys. I believe in that list. I also think it's worth being honest about where it runs out, and what an outside look adds that checking your own work, however carefully, structurally can't.
What a careful self-check is genuinely good at
Running the checklist yourself catches the mistakes that are visible once you know to look: is row-level security actually on, does a logged-out request to an admin route get refused, is a key sitting in the frontend bundle. These are checkable facts, and anyone willing to spend the twenty minutes can verify them without needing to hire anyone.
What it structurally can't catch
The limit isn't effort or carefulness. It's that you already know how your own app is supposed to work, and that knowledge quietly shapes what you think to try. You test the paths you built on purpose. Someone who didn't build it, and doesn't share your assumptions about how it's meant to be used, will try things you'd never think to try, precisely because they don't know it's not supposed to work that way. That's not a character flaw in you. It's the same reason writers can't fully edit their own drafts, or why a second radiologist sometimes catches what the first one's trained eye missed. Familiarity is useful for building. It's a genuine blind spot for finding what's wrong.
A concrete example of the gap
Say you built a feature that lets a user share a record with a link. You tested that the link works, that it shows the right data, that a normal visitor sees what they're supposed to. Someone reviewing it independently, with no attachment to how the feature is supposed to be used, might immediately try changing one character in that link's ID, purely out of habit, because that's the first thing an outsider tries on any URL with an ID in it. You might never think to try that on your own feature, because in your head, the link is for viewing your own record, not for guessing at someone else's.
So which one do you actually need
Run the self-check on everything, every time, regardless. It's free and it catches real, common mistakes. Bring in an outside look specifically once real user data, payments, or anything a regulator has a name for is involved, or once the app has grown complex enough that no single person holds the whole thing in their head anymore. The two aren't competing options. One is the baseline you should never skip. The other is what catches the thing the baseline was never built to see.