4) Writing well saves everyone’s time
Badly written documents tend to be long, dense affairs that take hours to read and often do more to confuse than to clarify. Instructions remain unclear so either the reader goes ahead and probably does the wrong thing or they have to go back and seek clarification from the writer. Either way this adds in time to the project. Someone somewhere has to pay for that wasted time.
The brief is unclear so either the company wastes time producing a proposal that fails to meet the mark or they have to go back and seek clarification. Again, poor writing adds in unnecessary steps. It makes business less efficient.
Think of any type of written business communication and in every case writing well would save time and increase efficiency. Of course it takes time at the beginning to learn how to do it well, and then it requires that little extra effort planning, crafting and editing the document to get it right first time, but it saves time in the long run.