The speaker may use an overt expression to strengthen the addressee's motive to carry out an instruction. This is the reinforcement of the addressee's reason to obey: the explicit name recalls why the prescribed act is appropriate.
فَإِذَا عَزَمْتَ فَتَوَكَّلْ عَلَى اللَّهِ
When you have resolved, put your trust in Allah.
Instead of a simple pronoun, «على الله» explicitly invokes the divine name and what it implies of power and reliable support. The rhetorical impact concerns the motive to trust, not merely repetition of a referent.