[slededit]

It broke up the relative monotony of otherwise dry technical information. It has a similar effect when placed in code. In addition many companies used it as a form branding - "Hey we're not stuffy old IBM! We're Cool!".

Back then manuals were also far more important, you couldn't operate the machine at all without one. Yet the jokes never managed to stop people from getting the information they needed.

I think the real difference when it comes to manuals is most modern ones are not comprehensive. I could see being upset if there was a joke but no information on my actual problem.