Here's why:
* Compound adjectives: "Hand-painted" is a compound adjective describing a noun (e.g., "hand-painted vase").
* Clarity and meaning: The hyphen helps to clarify that "hand" and "painted" are working together as a single descriptive unit. Without it, it might be unclear if you mean "hand that is painted" or something painted by a hand.
Example:
* "The hand-painted vase was a beautiful addition to her collection."