Hugo suggests, in a playful, poetic fashion, that butterflies are love notes written from one lover to another that somehow changed to butterflies.
As far as his purpose in doing so, it is hard to know an author's intent, but we can discuss the result of him doing so. It is one of countless ways to poeticize love. Like many of them, it links love and springtime. More specifically, it links the passion in the air of many springs with the actual pollination carried out by butterflies—it links love with reproduction, and verbal beauty with natural beauty.