Some people have poor handwriting which may not be easy to read. Writing the amount in two ways allows for verification.
The use of words as well as numerals presumably makes it harder to alter a check.
Written form means you have to write your answer in words, not numerals: ten, not 10.
The written amount missing on the check is the amount in words that corresponds to the numerical amount written on the check.
1942 IS written in Arabic numerals. MCMXLII is the same number written in Roman numerals.
4000 is written as MMMM.
In numerals : 1,150,000,000,000 In words: one trillion, one hundred fifty billion
1900 is written in Roman numerals as MCM.
In Roman numerals the number 77 is written as LXXVII.
The number 2100 written in Roman numerals is MMC
Seven tens or 70 is written LXX in Roman numerals.
The legal amount of a check is the amount written out in words. The amount on the check in numbers is the courtesy amount.
In terms of writing style, it looks and sounds better to use either all numerals or all words, so it is either 7700 or it is seventy seven hundred. Mixing numerals and words is a bit awkward.