While it's nice to think all jobs will be happy-go-lucky, relaxing jaunts through life, there will be times when the shit hits the fan and working under stressful conditions will be required. Possibly more often than not in some industries depending on the experience level of the employee.
Knowing someone can think clearly and make rational decisions during these times can be a useful thing to know.
Who are these people who actually do this? Over the last 25+ years I don't recall ever having had to write actual code on a whiteboard during an interview. I've been asked to describe solutions to problems out loud (and refine those solutions as the goalposts are moved), and been asked to describe what specific code fragments do, but I don't think I've ever written code on a wall. That's not how people write code.
What I have had to do in the past (and what I've asked someone to do for a technical interview tomorrow, as it happens) is write a solution to a problem before the interview and present and discuss that code in person. Code reviews are (should be) a regular part of the job. (Are you listening, Boeing?)