I've interviewed people for a junior position recently. I gave the FizzBuzz test, verbatim from Wikipedia. Only about 10% were able to do it on a whiteboard, without help.
TIL I'm a terrible programmer.
Whiteboards are for diagramming things more complicated than lines of code. [e.g. Interactions between several web services or paths of control across entire programs]
I've honestly just accepted that by the standards of many people's interviewing processes I'm a bad programmer. :)
Interviews are stressful. Keeping the candidate relaxed should be on the interviewer's mind.