Remaining within the tradition of narrating epic poetry, Spenser writes TheFaerie Queene with a (mostly) grand and elevated third person omniscient narrator who is clearly meant to evoke the author himself.
When adventures, great deeds, and solemn vows are being described, the authority and distance of a third person narrator can effectively communicate the solemnity of this subject matter. Since The Faerie Queene is also interested in providing the reader with moral lessons and advice, a third person narrator is a particularly effective way to make absolutely sure the reader is getting the point.
The narrator tends to interject himself in the opening few stanzas of every canto usually to offer a little moral recap of the canto previously and a little preview of the canto to come. In these recaps, the narrator is pretty explicit about the moral message he wants the reader to learn, sometimes even coming across a bit heavy-handed.
It's important that the reader has access to the thoughts, motives, and feelings of all the characters all the time so that we avoid being misled or deceived along with the characters in the poem and have access to enough information to make a judgment call ourselves. No pressure, of course. Having bad judgment in Faerie just means that, oh, you might be tortured, killed, or have your heart cut out by a lovesick demon.