Signup is the one journey where the people affected have no way to tell you. A customer with a broken checkout emails you, because they already know who you are. Someone who cannot create an account simply leaves, and you never learn they were there.
The confirmation email stops arriving. Registration succeeds, the account exists, and it can never be activated. From your database everything looks healthy - a rising count of unconfirmed users is easy to read as normal drop-off.
Validation gets stricter than intended. A password rule, an email regex, or a country field starts rejecting input that used to be fine. Often for a subset of users, so it never reproduces when you try it.
An OAuth provider changes something. A redirect URI, a scope, a consent screen. Google or GitHub sign-in breaks while email signup keeps working.
The final step 500s. The form submits, the user waits, and the request fails after the point where anything is shown to them.
The signal for broken signup is an absence: fewer new accounts. That is indistinguishable from a slow week, a seasonal dip, or an ad campaign ending. Nobody investigates an absence quickly.
And signup is rarely re-tested after launch. It gets exercised thoroughly the week it is built, then almost never again - while everything around it, from the email provider to the consent banner, keeps changing.
Four steps, none of which involve writing a test script.
Add your site, describe one journey, and let it run. Free, no credit card.