As far as I can see the reason for a large number of digits was to do the equivalent of floating point without having an exponent. Also a number of digits gets

As far as I can see the reason for a large number of digits was to do the equivalent of floating point without having an exponent. Also a number of digits gets lost doing differences and not doing proper rounding. Konrad Zuse was the first one to do floating point - and he had binary too in his mechanical computer, but it worked on completely different principles from Babbage's machines. Decimal is fine when one uses wheels, Zuse used plates where binary was much more natural. The digital mechanical multipliers I know of use rods for multiplying and wheels for the adding.