The trouble with whiteboards are that some people think nonlinearly. If I'm writing an algorithm, I may write the "middle" first. On a whiteboard, this means I have to guess how much blank space I need to leave in various parts of my code so I have room to fill it later. I almost always guess wrong. On a computer, this is not a problem.

Now while you may be aware of this and not give the candidate a hard time for not leaving enough space, I read a Medium post once written by a Google employee with tips on doing well on the interview. He explicitly highlighted this problem, and literally encouraged candidates to learn to think and write linearly and not leave these kinds of blank spaces.