Every wizard in The Lord of the Rings obviously had a purpose to be sent to Middle-earth, with Gandalf's being to help out in the defeat of Sauron and Morgoth's forces, etc. However, because Saruman was not returned to life after his death by Grima, he must have fulfilled his purpose.
I've read that Gandalf came back as the White in order to fulfill the purpose Saruman had failed in. So, obviously, Saruman had a purpose and failed it, but why? What was his purpose? And why was he not sent back to complete it, but instead Gandalf was sent in his place?