I have found many places that list the various rules on using hyphens in math, but nothing to explain why we have the rule. I have some students who are asking and I would like to be able to give them an answer instead of saying it is just the rule.
To avoid ambiguity.
He turned twenty one too many times.
Is not the same as
He turned twenty-one too many times.
This is rather similar to TsSkTo's answer, but why not see another usage?
Can I have twenty-five times?
This asks for the multiple 25 times (some unspecified amount or object) or perhaps it asks for twenty-five records of time.
Can I have twenty five times?
This asks for 100.
The hyphen prevents the former number from becoming an adjective describing the latter one.
EDIT: Changed the word cents to times to better match common usage.