If something is useful, you can use it to do something or to help you in some way.
例句
The slow cooker is very useful for people who go out all day...慢燉鍋對于整天都不在家的人來說很有用。Hypnotherapy can be useful in helping you give up smoking...催眠療法可能有助于你戒煙。
PHRASE
有用；派得上用場
If an object or skill comes in useful, it can help you achieve something in a particular situation.
例句
The accommodation is some distance from the clubhouse, so a hire car comes in useful.住處離會所有一段距離，所以租一輛車會很有用。
英漢詞典釋義
n.
有用,有益,有效
The index adds appreciably to the usefulness of the book.索引明顯地增加了這本書的實用價值。The report is of potential usefulness to the government.這個報告可能對政府有用。The theory is of limited usefulness.這個理論用途有限。Badly written questions limit the usefulness of questionnaires.問題設計得不好，調查問卷的用途就會打折扣。He has outlived his usefulness.他已老而無用了。