Why should I attempt to do all the homework problems?
You learn math by solving problems just like you learn
to play an instrument by playing on it. 

Why should I attempt to do the problems myself?
Recently, several people with prefect homework scores did badly on the exams.
It turned out that they had done their homeworks in groups. 
You get a good homework score this way but it does not 
help you getting a good score on the exams. You should first attepmt hard to
solve the problems yourself and then discuss them in a group.