expression, godan verb (-ru, irregular)
have business; have something to do; have something to say
Used when someone has a reason to visit, call, or speak to someone, often implying a specific errand or matter. Can be used for both urgent and non-urgent situations.
ちょっと用があるので、先に失礼します。
I have something to attend to, so I'll be leaving first.
彼に用があるんだけど、今忙しいかな？
I have something to talk to him about; is he busy right now?
何か用があるなら、遠慮なく言ってください。
If you have any business, please don't hesitate to tell me.
用事 is a noun meaning 'errand' or 'business', while 用がある is a verb phrase meaning 'to have business'. 用がある is more conversational.
用 is a noun meaning 'business' or 'errand'. 用がある is the full phrase for 'to have business'.
Phrase consisting of 用 (business, errand) + が (subject marker) + ある (to exist, to have).