[jacobolus]

An (analog) slide rule is a whole lot faster than doing digital arithmetic. But it’s approximate, yielding about 3 digits of precision, or maybe 4 digits on a large slide rule.

With an abacus or with written numbers, you can get 10 digits of precision (or 50) if you are willing to put the work in.

[mlyle]

You can iterate slide rule calculations to get more precision, too... You're not restricted to the width of your abacus nor the number of sigfigs you can get in a single slide rule calc..