Although it is often taken for granted, writing, and writing properly, is very important for every person. In all aspects, writing properly can make a person appear wiser than he really is, and also the opposite, writing incorrectly can give very bad impression of anybody. If you take into account that a large share of our communication is done in writing, you can understand you need to pay attention on how you write.
Writing properly is composed of three clue things, in my opinion: A correct use of the words of the language (grammar, semantics) and an understandable composition of sentences which would take the message from one extreme to the other might be the two most important ones; but using the correct style for each situation would be a good third aspect. Needless to say, every person should know how to communicate in writing correctly.
For this, the rules of thumb have remained the same for the last few centuries: reading quite much, because it will give you the grounds for different and more varied writing; and practising, because practice makes perfect. To this two I would add a golden third rule: simplicity. The reader must understand what you were trying to say. If he ends up with doubts, it means you didn't write properly.
Knowledge + Communication