When you have two objects in a sentence, the indirect object is often preceded by a preposition.
For example, the sentence ‘I couldn't figure out a way to convince him of my love', the verb 'convince' has two objects - 'him' is the direct object and 'my love' is the indirect object. The preposition 'of' makes it clear which is the indirect object. The fixed collocation is 'convince [direct object] of [indirect object] - in other words, 'convince someone of something'.
Different verbs collocate with different prepositions. For example 'supply someone with something' or 'recommend something to someone'.