A long series of single-spaced paragraphs with no heads is extremely daunting, especially to readers who do not know in advanced what the document is all about. Remember that professional writing is regularly read by people in a hurry who need to extract information quickly and who might need to refer to the writing again. So a document should advertise what information is where. Add heads, for example, to sections of a document that go together, and put long lists in column form. Clip art can also be employed to draw the eye to especially important ideas and sections.
Style is important because it can create a poor image of the writer if it's bad.
On the other hand, style can further a writer's purpose if it's good. Business and technical writers should state things simply and avoid trying to sound like business writers. If you are trying to sound like the writer of an important business letter or a really difficult technical manual, then the letter will sound phony and the manual will be unreadable. For example, if your document's subject is a technical one and your readers are insiders of the technical field, they expect the appropriate amount of technical jargon that outsiders to the subject would not understand. However, a document presenting a technical subject to non-specialists must avoid the jargon.