Usually we write technical documents to report results; that is, we write to communicate. But in addition, writing can serve as a tool for discovery : it can help make research more focused and productive. Nevertheless, it seems to receive less attention from engineers than other modes of discovery; at least, this use of writing is rarely taught explicitly in engineering schools.
Here are some of the ways writing can be used to support research:
The evolution of human culture parallels development of means for communicating: from gesture to speech, from spoken language to drawing, from drawing to writing, from written language to electrical transmissions (telegraph, radio, television), from simple electrical transmissions to computers. In all forms of communication the fundamental problem is the same: to transfer ideas from one person to another.
Similarly, abstract problem-solving requires a means for moving ideas from the mind into visible, tangible forms that can be analyzed and manipulated. The principal means for doing this is writing. In fact, Donald  has argued that ancient Greece became the foundation for modern Western culture because the Greeks discovered how to use writing to develop complex chains of reasoning.
To be most effective, the writing should be done regularly, as a systematic part of a research routine. It might be done via daily entries in a research journal; this would be more elaborate than a lab notebook, which tends to degenerate into a simple log of activities. Alternatively, the writing could take the form of weekly progress reports, status reports, or summaries, even though no one but the writer would be expected to read such reports.
As with any skill, this kind of writing improves with practice, and practice requires time and commitment. Skills must be developed and assimilated before they bear fruit, so you should not expect dramatic results immediately. But with practice, writing to discover can become an important tool in any researcher's toolbox.
E. B. White once wrote, "... writing is one way to go about thinking" . This mode of thinking can be used to advantage by scientists and engineers.
 G. D. Gopen and J. Swan, "The Science of Scientific Writing," American Scientist, 78, 550 (1990).
 Merlin Donald, Origins of the Modern Mind, Harvard University Press, Cambridge, MA, 1991.
 W. Strunk, Jr. and E. B. White, The Elements of Style, 3rd ed., Macmillan, New York, 1979, p.70.