Some teachers prefer to write the correct answers for students when marking their answer sheets. Some other teachers think that allocation of marks is enough because making corrections consumers a lot of time.
A teacher's duty is to guide the students. So they should write the instructions in the answer script so that the students can do well in their future exams.
For beginners it is ok to write the correct answers for them so that they learn better and know their mistakes. But for adults, I think it is better not to write correct answers and to make them search for the correct answers by themselves as that will help them to learn better and develop their autonomy in learning