The purpose of written communication is the same as that of oral communication: promoting mutual understanding between two or more people. If you were considered a bad writer by your teachers in high school, college or even graduate school, it does not mean that you are doomed to be one in your career. The same goes if your teachers told you that you were a good writer.
The writing that you will likely find yourself doing in your career is very different from the writing that you did as a student. Academic writing may have purposes, like self-exploration, developing ideas or just filling space. Writing in your career is about communicating specific information and viewpoints clearly with as few words as possible. You may have tried to write as much as possible to meet word requirements or page requirements when writing papers for class, but be warned that writing less is better in the real world.
Your boss does not want to read any unnecessary words and they will also expect the work that you do to be free of spelling or grammatical errors. In addition, selective use of bold, italics, and other formatting, and bullets can help order and convey a message more effectively. Always remember to take the time to carefully review and edit your work, and remember that the less ink you use, the better.