I once went into a music store and they sold an old studio tape recorder - I say old but it looked new, everything aluminium and all. The thing about that one is that it came with a manual that included all the schematics for the electrical components, so that you can repair, replace or rebuild the electronics if needs be. They weren't so worried about others copying them at the time, I guess - I mean it was a specialist product for a niche market, and just because you have the plans doesn't mean you can reproduce it in a cost effective manner.
Here in Brazil, in the 80's, (probably still happens today) you could buy suitcases filled with many pages of reverse engineered schematics of popular TV and radio sets.
If you had any problem with your TV, would could take it to a workshop, the technician could diagnose it, query the schematic of that specific model and replace any defective part.
There's so much of this. Think of how many perfectly good battery cells from powerbanks end up in landfills because of cheap micro USB connectors that snap off the PCB.
> doesn't mean you can reproduce it in a cost effective manner.
But you could fix it if, e.g. a capacitor blew. And capacitors do have a tendency to blow. Thankfully recapping something typically doesn't require a manual, since they are labelled, but you get the point. Repair is important.
Not just professional equipment, pre-90s every electric/electronic hardware itself larger than a book had at least a high level block diagram on a sticker on the back or on an inside wall, and also more detailed diagrams on manuals. Complexities then went overboard and those diagrams became rarer.
I remember those diagrams also didn't include mechanical drawings, so the last part of your comment is right too. You aren't going to be making magnetic read/write heads or even DC motors from information taken from those diagrams, merely the topology of control circuits - which is an opposite view from how we'd see it today.
My TEAC A-X55 Mark II sound amplifier manual comes not only with the full block schematic, exploded view but with the full BOM and even all the PCB printouts... you can basically build the thing from scratch: https://imgur.com/a/hZeWFUF
