It’s slightly depends, but essentially nothing.
Predicted grades are just predicted grades based on what you’ve done before. In the actual exams soo much can happen and you might not achieve them.
If you don’t have the actual grade outcomes yet, and only predicted ones, then you put those in. If you’ve not done the exams or essays yet, then the predicted grades are what you have.
If, after you get the grades outcomes, they ask you what grades you got eg in the interview, then you should tell em. (If you didn’t end up getting the grades but no one asks you, best not to raise it).
Very rarely, if ever, but if the job offer is based on certain grades in certain subjects, say like a uni offer when applying for undergrad, then they might take any job offer back or not end up hiring you but this would be clear in the application process.
Like I said, that’s rare and I’ve never heard it happen to anyone but technically it’s possible.
Usually, predicted grades are used as criteria to help with shortlisting (I.e. easier to exclude people who are predicted low grades) and like I said just an indicator to how capable, smart etc you might be for their “prestigious” entry level role.