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.
I'm thinking this is a little too late for me to be doing your school work for you...
but for every one else wanting the answer, the poem is about lost love...
Butterlies originate in his poem from ripped up love notes from his past lover. Basicly, he's ripping up these notes on a somewhat windy day watching them fly and take air. Resembling butterflies.
Basic but to the point