When I was in school to become an mechanical engineer (1970 to 75) my first two years we were required to use slide rules because hand held calculators were too expensive for the average student, and the affordable ones had just 4 functions, which couldn't do the same math as a slide rule. By the time I was a Junior, the HP35 had came out which had enough functionality to actually replace a slide rule. Some of our professors allowed them to be used in tests, others didn't.
Slide rules are easy to use once you learn how, and quicker than entering numbers on a calculator. And you aren't as apt to dial in a false accuracy with a slide rule (significant digits were a big deal to most of my professors). It is also much easier to deal with and visualize logarithmic relationships. I actually used one for some of the questions on my PE Exam a few years later.
I still have one but am not as nearly as proficient with it as I was back then. In fact, I have to scratch my head quite a few times to use it.