Back in the time of Gauss, mathematicians needed logarithm tables, and the only ones they trusted making them where mathematicians. So, mathematicians computed logarithms 'in their spare time'
He also had to compute prime numbers so that he could study their properties. (http://science.larouchepac.com/gauss/ceres/InterimII/Arithme... claims he extended a table listing all primes up to 10,009 but never got as far as a million, but he must have spent lots of time on calculations we now call mundane
For example, Gauss actually spent time to write a review of a table of logarithms (http://articles.adsabs.harvard.edu//full/1851AN.....32..181G...).
He also had to compute prime numbers so that he could study their properties. (http://science.larouchepac.com/gauss/ceres/InterimII/Arithme... claims he extended a table listing all primes up to 10,009 but never got as far as a million, but he must have spent lots of time on calculations we now call mundane