Why does the US law sometimes mention numbers in both digits and letters?

Question

Why does the US law sometimes mention numbers in both digits and letters?

E.g. https://www.govinfo.gov/content/pkg/CFR-2012-title8-vol1/pdf/CFR-2012-title8-vol1-sec316-5.pdf:

Absences from the United States for continuous periods of between six (6) months and one (1) year during the periods for which continuous residence is required under §316.2 (a)(3) and (a)(6) shall disrupt the continuity of such residence for purposes of this part unless the applicant can establish otherwise to the satisfaction of the Service.

Typically people don't write the same info twice.

Answer 1

This is very common in all sorts of legal documents, not just the United States Code. Another familiar example where this is seen is on checks.

It serves as a sort of "redundancy check", to help catch errors where either the words or the numerals could have been incorrectly transcribed. Of course, one could ask why similar redundancy isn't used to avoid errors in other contexts; there doesn't seem to be a good answer for this besides "tradition".

Some people feel the practice is obsolete and should be abandoned, e.g. https://www.butlersnow.com/2020/04/five-5-reasons-to-stop-writing-numbers-like-this/.