Simple: I get to say “Good morning/afternoon, ___, how are you today?” to every student. I always greet classes with “Good ____, how are you?” but that doesn’t give everyone a chance to respond because the loud voices drown out the quiet ones. But asking the question one-on-one means I get to say hello to the students I can’t always say hello to. It allows us a warm-up chat before the speaking assessment begins. It’s been my experience that Korean students to get visibly nervous before speaking tests, so perhaps my asking “How are you today?” allows the students a gentle chance to focus their minds on speaking in English as well. “How are you?” is a question they know well, so they can start off knowing they’re off and running.
“How are you today?” gets asked for another reason: I want to show the students I care about them and am curious about what’s on their minds. People tend to perk up when others are interested in them; taking an interest in students can help their well-being. School can be a lonely place and I've endeavored to make it a little better with every lesson I do. It’s easy for the students to get lost in the crowd and get ignored—indeed, I struggle with names and faces at times, but thankfully, with the student roster in front of me, I can use the student’s name, smile, and begin the conversation. I've no way of proving it aside from first-hand observation, but these quick conversations help set the students up for success in their speaking assessments.
*See Dale Carnegie’s classic How to Win Friends and Influence People for more on this subject. It’s a book I’d recommend for anyone who’d like to better talk with and understand people.