on what business; what brings you here
A set phrase used to ask someone's reason for coming or purpose of a visit. Often used in formal or polite contexts, but can appear in everyday speech.
本日は何用でお越しですか。
What business brings you here today?
すみません、何用でこちらに？
Excuse me, what brings you here?
Compound of 何 (what) + 用 (business, errand) + で (particle indicating means or reason). The exact historical derivation is straightforward.