I think the main problem with &quot;coding on a whiteboard&quot; during interviews is not that you have to reason about an algorithm without the help of a computer, but rather that you have very limited time to do so. You stand there, looking at an empty board, trying to come up with something sensible (or at least, a decent start) in a few seconds. In the meantime people are looking at you, waiting for your answer, which adds just a tiny bit of psychological pressure. :) This is a very different situation from sitting down at your desk and thinking an algorithm over, at your leisure.