Slide rules were wooden tables of logarithms, about 30cm long, built with appropriate technology to have only two moving parts (a slidey bit and a cursor to help line things up) and the batteries never run out.
Now here's the rub. You use the slide-rule in exactly the same way (and get the same answer!) whether you are multiplying 2 x 3 or 20 x 3 or 200 x 3 million. So you had to estimate the answer you expected before you started tricking about with the apparatus. When you read off the significant figures from the slide-rule, you knew whether you were talking 6 or 60 or 6/10ths of a billion. I just showed how to do this by estimating the seconds in a calendar year. So with the tools of more than 40 years ago, you knew how to ball-park, got experience in estimating and developed a feeling for numbers and magnitude. The calculator generation tend not to have this skill particularly well polished. The other great advantage is that you're not tempted to write down an answer like 3.1415926 for Pi, when 22/7 is good enough for your present purpose.