What was the significance of Portia's song in act 3 scene 2 of The Merchant of Venice?

The song is designed to help Bassanio make the right choice in the casket test.
There are three caskets involved in the test, but only one will lead to Portia. Portia desperately wants Bassanio to choose the correct casket, but she can't come right out and tell him which one to pick. So she does the next best thing and gives him some none-too-subtle clues in the song. The last word of every line in the song rhymes with "lead," which is the metal from which the prize casket has been made. The other two, made of gold and silver, were designed by Portia's father to throw fortune-hunters off the scent.
It's not entirely clear that Bassanio picks up the hint, but in any case, he does choose the lead casket, which for Portia is ultimately all that matters.