Technical illustrating, like technical writing, bridges technology to the people that use it. Technical illustrating allows the user to not only see what a component looks like, but also how it works, how its parts fit together and how it interacts with other components.
Technical illustrating, incidentally, is what I enjoy doing the most (Not that I don't enjoy doing the other things too!) and I particularily welcome assignments that involve technical illustrating.
Technical illustrating can be a very effective and efficient method of technical communication. Often, what would require a lot of writing to describe can be conveyed in a glance by a good technical illustration. Writing has limitations of language and syntex, but imagery does not. In this global economy, have you noticed that instructions and explanations that come with products tend to use more images and less writing? It's the same with the visual medium of the internet: pictures and images are what capture attention.
My technical/artistic background allows me to understand how something works and then illustrate it, and a lot of my work experience has been exclusively as a techncial illustrator or graphic artist. In all of my technical writing jobs during the last 20 years for corporations and businesses, I was also the technical illustrator. Not only can I create technical illustrations, I can also complement words with illustrations (and vice versa) in explaining technical concepts.