Oh I actually might know the answer to that one! According to my Commercial Paper professor it was an early way to prevent fraud and mutual mistake. It’s fairly easy to sneak an extra zero on the end of a written number or to transpose two numbers without anyone noticing. Checks still do this: “Pay twenty and xx/100 dollars” followed by “$20.00”

In theory if the written number doesn’t match the numeral the teller shouldn’t accept the check.