To have business/errands; to have something one needs to attend to or something to say to someone. Commonly used to give a reason for leaving or contacting someone.
すみません、ちょっと用があるので先に帰ります。
Sorry, I have something to take care of, so I'll go home early.
先生に用があると受付に伝えてください。
Please tell the reception that you have business with the teacher.
電話をかけたのは用があったからです。特に相談したいことがあります。
I called because I had something to discuss; I especially want to consult about something.