In the following sentence, why is there a direct object pronoun "l'" after "si"? Is it only for the purpose of euphony (sounds better than "si on"), or is there a grammatical purpose I'm missing? It doesn't seem necessary because there is already the pronoun "on."

"Si l'on apprenait qu'il ne nous reste qu'une journée à vivre, ce qui nous paraît catastrophique deviendrait soudain très relatif."