Why pray to angels?
The short answer
Angels are spiritual beings with free will and intellects far beyond ours. The holy angels dwell in heaven and so are saints. God uses them as messengers, guardians and helpers to us, as Raphael's words in Tobit show.
Angels are spiritual beings with free will and an intellect far superior to ours. The evil ones are in hell; the holy ones are in heaven and so are considered saints, since all who are in heaven are saints. God uses angels as messengers, guardians and all-around helpers to us, and to reflect his glory.
In the Book of Tobit, the archangel Raphael tells Tobias that when he and Sarah prayed, it was he who brought the record of their prayer before the glory of the Lord, and likewise when the dead were buried (Tobit 12:12). He says God sent him to heal them, and reveals himself as Raphael, one of the seven angels who stand ready and enter before the glory of the Lord; the two fall face down in fear (Tobit 12:14-16).
For more, see the Catholic Encyclopedia article on angels.