Everyone involved in the process wants you to succeed. If not out of the goodness of our hearts, then because we've invested time. Under-preparedness is a stupid and preventable reason to lose a candidate.

When someone comes in and bombs, that is at least valuable information: we don't want to work with this person. When someone comes in and seems smart and capable, but makes hardly any progress on the problem because it's obviously their first time on a whiteboard, it's just regrettable all around.

We're not selecting for preparedness. In fact, we try to select against it: interviewers take pride in designing twists that separate understanding from recall. If you seem to be recalling, we're supposed to pick a different question, or failing that, report a null signal. But we do need you to be prepared enough that your interview skills don't get in the way of your actual skills.