I think what Hugo is trying to do is to combine the immense joy that he feels at the beauty of nature with a paean to romantic love. As he watches the little butterflies flutter from flower to flower, he's instantly reminded of scraps of paper from tattered old love letters scattered by the breeze. Springtime is traditionally associated with love and romance, especially in poetry. In the words of Tennyson, it's a time of year when "a young man's fancy turns lightly to thoughts of love." It's also a time of great natural beauty, when the whole of nature seems to burst into life after its winter slumbers. Hugo combines these two elements of spring in a wonderfully whimsical and inventive conceit: that butterflies originate from the torn scraps of love letters written on soft silk paper.

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.