Not to grade you. To stop you spending twelve weeks silent in a class that matches your paper rather than your mouth.
Almost everybody arrives at the placement test expecting to be judged, and the assessor spends the first two minutes saying that nothing here goes on a record and nobody fails it. Then we do the test anyway, because the forty five minutes prevent the single most common way a language course is wasted.
Here is the failure it exists to prevent. Somebody learned English at school, mostly on paper, and reads and writes it reasonably. They sit a written test, score well, and go into level four. In week one they discover that everybody around them can talk, and for the next twelve weeks they say almost nothing, because saying something would reveal the gap. They attend sixty hours, they learn very little, and at the end they conclude they are bad at languages.
So the test has two halves and the second one decides most cases. Twenty five minutes on paper tells us what you have been taught. Fifteen minutes of unhurried conversation with an assessor tells us what you can use. Nobody is timed on an answer and there is no right way to sound.
Where those two halves disagree, and they often do by two whole levels, we place by the conversation and we tell you that is what we are doing. Some people argue, reasonably, that their reading is wasted in a lower class. The answer is that the lower class will move faster for you than it does for anybody else in it, and that twelve weeks of speaking is what you came for.
You are told the level out loud on the day, with the reason. If you are between two levels you are told which way we would go and why, and then it is your decision, not ours.
And sometimes the answer is that the course you asked about is not the right one at all. Arabic conversation assumes you can read the script; if you cannot, we say so and we do not take the fee. That is a worse afternoon for the office and a much better ten weeks for you.