Why Writing a Check Requires Spelling Out the Number
Writing a check's amount in words as well as numerals exists as a fraud-prevention and error-catching measure — the written-word amount is meaningfully harder to alter after the fact than a numeral (which can be edited by adding a digit, for example, changing $100 to $1,000 more easily than the equivalent words could be convincingly altered), and by long-standing banking convention, if the numeral and the written-word amounts on a check ever disagree, the written-word amount is treated as the legally controlling one.
This dual-format requirement traces back to a genuinely practical security concern with paper-based payment instruments, not just tradition for its own sake.
Why words are harder to tamper with than digits
A numeral like "100" has limited characters and simple structure, making it easier to alter convincingly by adding a digit or changing a single character — a fully spelled-out amount like "one hundred and 00/100" is a longer, more specific string of text that's considerably harder to alter without the tampering becoming visually obvious.
Why the written amount legally controls in a mismatch
Since the written-word amount is the harder of the two to fraudulently alter, banking convention and commercial law in many jurisdictions treat it as authoritative if it disagrees with the numeral — which is exactly why accurately spelling out the amount matters as much as, or more than, getting the numeral itself right.