Question

The hyphen in "one-time winner" is used because:

Answer 1

if you write one time winner (without the hyphen) it's sort of suggesting that the person is winning time

if you put the hyphen it makes it clear that one is modifying time and the whole word "one-time" is modifying winner, saying that the person won one time

it also alters the pronunciation to put the emphasis on "one" when you say it (ONE-time WIN-ner) rather than "one TIME winner)