Email verification is a complicated process, so there will be some instances where we are unable to fully verify the email we've guessed.
Catch all means that the emails we guess will be deemed as correct by the domain you're trying to contact. Which means we cannot be 100% certain that the email tested is the correct one. In these cases there is a higher probability of the email bouncing.
In order to avoid this situation we use third-party tools that allow us to have higher confidence when predicting the email.