I'm referring to the peculiar habit I sometimes see in formal documents, where a number is given numerically after it's spelled out. It seems quite redundant:
I need five (5) kumquats, stat!
(Though it isn't always in a legal context, might it be related to this answer on this question?)
The redundancy is presumably intended to reduce typos, forgery, and other types of error or manipulation in order to make the intent as clear as possible. It is known professionally as a CYA.
It's obvious what my intent is when I rather carelessly write an I.O.U. for sevum kumquats, but alas, sevum is not really a word and opens up our contract to litigation. Adding the numeral 7 in parenthesis after sevum clarifies the intent. Similarly, if I were to write an I.O.U. for 3 kumquats, one could easily change that 3 to an 8, but it is far more difficult to make three look like eight. The practice appears to be a legal tradition rather than a universally honored rule.
If you've ever written a check (known as a cheque to the Queen), you've written out a number with words and followed it with a numeral (albeit without parenthesis).
I am given to understand that this convention is not a best practice in legal drafting. Like many legal drafting convention, it is an arcane practice that persists long after its useful life, because lawyers don't question their style conventions enough. It came from the days of handwritten contracts, where it would have been easy to fraudulently alter a numeral. Writing out the words was a failsafe. This is why the convention is still done on checks -- they are handwritten. In today's world of document processing, it's no longer necessary, even dangerous, to use this convention in contract drafting. If there is a discrepancy, the words control, and they are harder to proofread. An example would be: one million dollars ($100,000). The eye picks out the number, not the words, but the words control. I learned the above from Bryan Garner (now editor of the Redbook and Black's Law Dictionary) though I cannot find a citation for it. I also seem to recall there was a famous dispute involving a New York law firm that turned on such an error. In any case, I don't use the convention in my documents; instead I use AP style.
The above answers in toto seem to comprise a correct response. In literary terms, the practice is the equivalent of the old burlesque show advertisements hawking "Twenty girls - count 'em, 20! - onstage."
This is a guess, but in old hand-written legal documents where paper/velum was a a premium, you couldn't afford to format a document with spaces and paragraphs and such to increase readability. What they did was to write key phrases like TO HAVE AND TO HOLD in all caps and large to stand out. I have heard this was to aid in finding relevant portions for reading. Perhaps the number in parentheses (or brackets as my people call them) is just a way to draw the eye quickly to the relevant figures when skimming?
In my job, I've prepared documents this way for several years, and there has never been any explanation other than basically what @Snumpy says: It's to reduce the chance of anyone's being able to claim in court the amounts were unclear.
It may, as @Sam mentions, date back to handwritten legal documents. Maybe people figured handwritten numerals were less likely to be misunderstood than handwritten text. It may also relate to the fact that, in earlier times when fewer people were literate, there were many illiterate people who understood numbers and could perform calculations.
I looked into this myself recently and couldn't find a definitive answer. The best I came up with was that numeric values in a formal document or contract are often crucial to its purpose. Restating a number in parentheses after spelling it out is a way to ensure the reader that the number is correct. It also draws the eye to the numbers, allowing for quick perusal of a document, say a purchase order.
These are practical reasons; whether there is some historic reason for it I do not know.
