Playtesting is incredibly valuable to a game designer. Since a designer is only one person, they are inherently biased towards their work. They might think their game is completely understandable, fair, interesting and fun, when in reality it might not be. This is where playtesting comes in. Having someone else (preferably a member of the game’s target audience) play the game as early as possible will help the designer have a better comprehension of what parts of the game need to be clarified, adjusted or possibly removed entirely. Game design is all about iteration, and playtesting between significant iterations is helpful to validate whether or not the changes being made to the game are improving it.
Below is a flowchart I created to help guide a playtesting session. You, the designer, are the Proctor (the one giving the playtest) and your role is to simply observe the tester and record as much data as you possibly can about the Tester’s emotional status, what they are finding frustrating or trivial, what they’re failing to understand, how they make decisions, and of course figuring out the WHY to all of those questions.