It indicates indirect object. In English you would just say "I bought Juan a car", but an unnatural translation might be more helpful: "I bought a car *to* Juan".
Technically, you shouldn't need "le" in cases like these, but nearly all native Spanish speakers would construct the indirect object this way (redundant/duplicate indirect object). My Latin teacher was an exception and he would always say "compré un coche a Juan", "escribí una carta a mi madre", "la madre dio un regalo a su hija". Either that or he would omit the noun (like most native speakers do): "le compré un coche", "le escribí una carta", "le dio un regalo". But he'd never duplicate the indirect object.
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