Why summer reading?
Why do English teachers assign summer reading assignments?  

In November of 2007, the National Endowment for the Arts completed a study revealing that: reading and reading comprehension was one of the top skills employers valued in employees today; good readers generally have more financially rewarding jobs; less advanced readers receive fewer opportunities for career growth; and struggling readers tend to be far more likely to drop out of high school.  In order to combat these findings, students need to read even when they are not in school.  

In 2000, the US Department of Health and Human Services completed a study on teaching children to read which showed reading and writing develop concurrently and interrelatedly.  This means that if a student stuggles with reading, they will also struggle with writing (not the mechanical motion of writing but the ability to actually express one's thoughts coherently in writing).  Thus, students that improve their reading abilities will also improve their writing abilities.

Bottomline: Students need to practice their reading as if it were a sport they were practicing for because it is only with practice that a person can improve their abilities.