Playtesters are in charge of testing gameplay for player experience. The main purpose of a playtest is to see if the video game is experienced as intended—if the controls are intuitive, the mechanics work as designed, the player can understand the purpose of the game, or if it is even fun to play.
Play testing is a fun way to get your foot in the door and gain some experience in the gaming industry. And who wouldn't love to play games all day and get paid for it?