In our industry, sketching is everything, we often revert to a simple sketch to show or explain a detail,. You will here many professionals quote " Words fail see sketch ", and out will come detail paper and pencils, or perhaps ink.
You may be asking why use a T square, well it's all about training the mind to think as a technologist, understanding a detail, the sequence of operation or the way materials are laid down or used.
These slides are my lectures on sketching, drawing, detailing, all done before a computer is switched on