They existed because the manufacturers would not always release the factory schematics or service manuals for mass-produced consumer products.

This proliferated once consumerism & disposable-ism really took off, before that most electronics came with a schematic, but it was still not often up to the standards of places like HP or Tektronix.

So Sams took what info they could get and reverse-engineered the rest as they tore down and scrutinized new equipment to produce these fairly standardized publications that were available by subscription to independent repair operators, covering each popular model from many different manufacturers as they emerged in the marketplace.

A lot of the repair operators were very familiar with gear like HP and Tek and so was Sams.

Most of them had successfully leveraged that kind of top documentation and serviceability to have materially contributed to the winning of World War II. Slouches they were not.

There was a preference not to want to settle for less in consumer electronics so they published their own to make up the difference. If the skills are there why not use them? Especially when a failure is usually only one or a small number of the most common dirt-cheap components that need to be replaced to fix.

Which is what electronics servicing is supposed to be all about from the beginning.

I know it's not the beginning any more but still there was a time when the pros had in the back of their mind the kind of things it might take to win World War III, more so than now.

With people like that around you didn't need right-to-repair laws to be enacted, the whole country was still aware of the benefit of field repair if not improvisation.

All of the overseas Communists were certainly not going to sleep on it.