the method of slips is the only one mechanicallypossible for the purpose of forming, classifying, andutiUsing a collection of documents of any greatextent. Statisticians, financiers, and men of letterswho observe, have now discovered this as well asscholars.
Moreover
A zettelkasten type note taking method isn't only popular and useful for scholars by 1898, but is useful to "statisticians, financiers, and men of letters".
Note carefully the word "mechanically" here used in a pre-digital context. One can't easily keep large amounts of data in one's head at once to make sense of it, so having a physical and mechanical means of doing so would have been important. In 21st century contexts one would more likely use a spreadsheet or database for these types of manipulations at increasingly larger scales.