。
She spends a lot of money on shoes.
Literal
She [topic-は] shoes [on-に] a lot of money [object-を] uses.
The construction Xに(お)金を使う ('spend money on X') uses に to mark the target or recipient of the spending. It parallels English 'on' but is structurally a target marker — the money is being directed at X. Note that 金 here is read かね ('money') without the お~ honorific that's often added in polite speech (お金) — bare 金 can sound a touch blunt or money-grubbing depending on context.