It's a filter, but not a particularly effective one. It just shows a human can act like a robot and memorize/regurgitate information. It doesn't show dynamic problem-solving skills (nobody at a real job is pressured to solve a complex problem in 30 minutes on a whiteboard, and most people should be asking their teammates or researching to find the best answers), interpersonal skills, communication skills, or the specific skillset needed for a specific role. You have no idea how this person is going to adapt to those challenges just by showing you an example of a linked list. It's also possible that interview challenge questions are the only thing the candidate knows.