Why do lecturers give only couple of days to do exercises? I have always been on a courses where lecturer teaches something and then gives the exercises on that subject. Is that a better way than reveal all exercises when the course begins? I mean, why I should always have only one week time to write solutions and then wait until the next set is revealed? I understand that this gives equal amount of time to think every problem but is it always necessary, as people learns by various speed?
The instructor may not want the students to work far ahead in the class.  

If the student does go on ahead and is doing exercises incorrectly (due to lack of knowledge of the subject), then it is may be harder for the student to learn the subject matter correctly due to the bad habits he or she has developed.