Generally, parents can motivate their children to perform at a level at which the child is capable. Parents tend to motivate their sons/daughters by taking away cellular telephones, vehicles, all-terrain vehicles, televisions, or other items which teenagers tend to like.
When parents take away these items, students get the message that if they want their prized-possessions returned, they need to hit the books and pull up their grades.
On the other hand, parents are tax payers, and if you are a teacher in a public school, parents pay your salary. Parents have a right to know how their children are doing in your class. If you keep parents updated throughout the year, parents tend to be supportive of your efforts, and you will avoid those upset parents at the end of the semester. In turn, your life is a bit less stressed, and you do not have any parents wishing bad things to befall you.