Firstly, writing assignments require you to interpret the concepts learned while working problems.  Expressing ideas in your own words helps you to explore your understanding of a subject and allows your instructors to evaluate your progress.  Secondly, chemistry is a vast experimental science that is only possible by many scientists recording and transmitting their findings in writing to other scientists.  This process requires a methodical and consistent style of writing that may be very different to other writing you may have encountered.  Good chemical writing should be clear, precise, well organized, and present statements supported by evidence.