I work in R&D so a log book is priceless. I make sketches, circuit diagrams, code notes, test results, calculations, ideas etc.
I date everything because that can be useful in R&D if trying to prove an idea has been stolen in later years.
I have recently started making an electronic summary in OneNote. One of my gripes about keeping log books is that it is difficult to find old notes (when you use up to 10 log books per year). OneNote does a good job for documenting work, it is easily searchable and multiple users can add to it (if working on a project).