I understand that the translation of the Hebrew word מלאך means "messenger" and an angel is "G-d's messenger."
We see angels mentioned in various roles. Frequently, angels appear in dreams. Sometimes, they are assigned tasks such as the three angels that appeared before Avraham.
If G-d is present everywhere, and he is able to do everything, even doing many things simultaneously ("multitasking"), then, why does G-d need to send angels or messengers? Why doesn't G-d just do the job, himself?
In the דרך לחיים edition of דרך השם, at the very end of חלק א, there is a footnote (#12) in which R' Chaim Friedlander addresses this question:
בודאי הקב"ה אינו צריך את המלאכים כדי להוציא לפועל את הנהגתו והשפעותיו, והיו באמת מקרים שהקב"ה הנהיג בעצמו בלי אמצעי שנקרא מלאך, כמו שאנו אומרים בהגש"פ "אני ולא מלאך... אני ה' ולא אחר" אלא "שכן רצה וסידר" שיהיו חוקים "שתהיינה גזרותיו יוצאות למעשה על ידי מלאכיו" וזאת משום שהקב"ה רוצה שנדע את הסיבות איך הדברים יוצאים לפועל כמו שבחוקי הטבע יש כללים שלולא היינו יודעים אותם לא היינו יודעים איך להשתמש בדרכי הטבע, כך הוא גם בחוקים של הרוחניות שבלי לדעת אותם לא היינו יכולים להפעיל אותם, כגון: שמצוה זו מפעילה מלאכים מסוימים להשיג תוצאה פלונית, כי הקב"ה מנהיג את הועלם לפי הבנתנו ומעשינו, וזה מה שאמרו חז"ל (אבות ה:א) "בעשרה מאמרות נברא העולם... והלא במאמר אחד יכול להבראות" עשרה המאמרות הם עשרה שלבים והשתלשלות, שאז אפשר להבין סיבה ומסובב, ואם היה נברא במאמר אחד לא היינו מבינים כלום בהשתלשלות הבריאה וסיבותיה. "אלא להפרע מן הרשעים... וליתן שכר טוב לצדיקים" כי הצדיקים מכח העשרה מאמרות מבינים דרכי שמים ופועלים על פיהם ועל כך מקבלים שכר, ואילו הרשעים שיכולים להתבונן ולהבין, ואינם רוצים ללמוד ולעשות את רצון הבורא, ועל כך יענשו
Paraphrased, R' Friedlander essentially says of course Hashem does not need angels, and sometimes He does act without them. But just as Hashem made the physical world with a multitude of laws, in a way that we can understand, so that we should be able to appreciate how they worked and to utilize them, so too Hashem made Spritual "laws" in such a way that they are in an orderly system that we can understand and relate to. There are therefore Spiritual rules of "cause and effect" so that we can appreciate how, say, a given Mitzvah instigates certain angels to effect a given result. By our being able to appreciate how our Mitzvos affect the world, our performing them becomes that much deeper. Angels represent that relationship of things being carried out, and there being cause and effect, which enable us to understand the spiritual structure of creation.
The answer is that G-d created the universe for our benefit not His. Malachim - messengers parallel a king and his ministers, servants etc. The King Himself doesn't deal with day-to-day tasks because it is beneath him. When we understand that idea, then we also understand that when G-d Himself deals with, and is directly involved with the world, it is for the Jewish people - His beloved (Shir HaShirim). With that in mind, we can perhaps understand that malachim simply express our relationship to the King of Kings. Sometimes we relate to Him directly, and sometimes through His servants. When we do Mitzvos, learn Torah and behave correctly, we interact with G-d on a level that is, to some degree direct. On the other hand, the nations of the world, and when we're not zoeche (have the right merit), we relate to G-d indirectly and hence, we relate to Him via His servants - the malachim.
The Ramchal, in Derech Hashem (I 5:3) explains in a far more detailed manner that Melachim are there to sustain and serve physical things so that they may serve and direct themselves to the Will of G-d.
In this respect then, since G-d's overarching presence is limited to some degree (we would simply not exist otherwise - being totally absorbed by the Oneness of the Creator), in order for us to relate to that, Hashem created spiritual beings that help us relate to the spiritual world.
While this does not answer why but hints on consistency through the creation:
If we put all the entities in the creation on a "spirituality axis" we could spot a pattern of different proportions between the material and the spiritual.
So angels seem to fit the picture perfectly, filling a place between us and some "pure spirituality".
PS: I was hesitating a lot about placing "humans" and "Jews" and the 50/50 breakpoint. It is not final, just a proposal.
