Narrative analysis: The story is moved along at the beginning by a guide. Eventually the guide fades, but the introduction is lead by the guide. This role occurs in both the Joseph Smith Sr. dream and in the Narrative of Zosimus ("and behold and angel of the Lord was sent saying to him, Zosimus, man of God, behold I am sent by the Most High God of all, to tell thee that thou shalt journey to the blessed, but shalt not dwell with them" Narrative of Zosimus p. 315). The guide serves to announce the beginning of the dream, as well as to indicate that the dream is coming from a higher source, that is, the source from whence the messenger came.
Symbolic analysis: In Lehi's dream, the messenger is in white, which color symbolism in the Book of Mormon matches the Western usage of pure, clean, and in the case of a dream, divine. Thus a messenger from God (angel by definition) comes to lead Lehi on the journey.