Sometimes an "easter egg", especially if not blurred out because of copyright issues.
Sometimes it's one of the directors previous work, or something the lead actor was in before, or a reference to a different series or an actor like a joke or something.
But in this case my guess is that is younger hairier Lee Jong Suk in Running Man from 2010.
Not sure anymore.
https://kisskh.at/people/900-lee-jong-suk