We have a great FAQ article on the subject [img]smilies/biggrin.gif[/img]
Their motives would have been, depending on what version you contemplate, to help the peoples in the East as a part of their mission, or to seek power of their own there. At any rate, their mission was to deal with Sauron, and we do not know anything beyond that. Sauron proved more than enough to handle already.