Seconded. Another factor is that looking at a device won't tell you why it was designed the way it was. A good technical manual will be invaluable for telling you things like valid ranges, service or environmental limits, etc. which would otherwise need to be reverse-engineered. If you're trying to replace an old system or studying the history, know what does and doesn't matter can save a ton of time.

(This goes double if the manual was annotated by a good operator)