2. Hashgacha
Another reason we have nevua is to show that Hashem is with us and watching over us:
והסיבה השנית למציאות הנבואה, כי גם שמצד השכל יושג מציאות השי"ת והשגחתו וישרו בהנהגת העולם, רצה האל ית' להשפיע שפע נבואיי על המוכנים כדי שיעידו על מציאותו יתברך ושעיניו פקוחות על כל דרכי בני אדם לתת לאיש כדרכיו
וכפרי מעלליו.
וכפרי מעלליו.
מבי"ט בית אלהים שער היסודות פי"ט
The second reason for the existence of prophecy is so that even intellectually we understand that G-d is running the world. So G-d wanted to bestow prophetic powers on those who are prepared in order that they should testify about the existence of G-d and that His eyes are vigilantly watching all of the deeds of man in order to give each person what he deserves.
I will say paranthetically that what gives me a big chizzuk in emunah is reading the writings of Rav Kook and other tzadikim who describe supernatural experiences they have. Please see this remarkable article. It brings home the lesson that He is really watching.
The Rambam in Yesodei Hatorah [10/3] writes: We see from this that a prophet will arise for the sole purpose of telling us the future events which will transpire in the world, whether there will be plenty or famine, war or peace, and the like. He even will inform a particular individual regarding his needs. Hence, when Saul lost an object, he went to the prophet to discover where it was.
When you go to a Navi, you see through him that Hashem is watching.
The Vilna Gaon [Mishlei 16/4] explains that a person would go to the prophet in order to find out what his purpose was in this world. That is the meaning of the pasuk לאדם מערכי לב ומה' מענה לשון - Man sets his heart with the desire to fulfill G-d's will, but מה' מענה לשון - the answer comes from Hashem via the prophet. Wouldn't it be GREAT if we could go to a person and he could relay the Divine message of what our job is in this world, where we should live, who we should marry etc. etc.
The Ramban in Vayikra [26/11] says that in the olden days they didn't have any use for doctors - if you have a medical problem, go to the Navi and he will tell you what to do in order to take care of it.
The Kuzari [1/ 103-109] has some very beautiful passages where he describes the pleasures of prophecy. Prophecy is living proof that Hashem dwells among us. Other religions talk about reward in the next world. We have prophets who receive their reward in this world as well when they merit beholding the Divine right here in this world:
הן כולן מבטיחות את השכר לאחר המות ואילו בחיים אין בהן מאומה ואף אין דבר שיורה התאמתותן של ההבטחות. הבטחות השכר שהבטיחה לנו תורה זמן התגשמותן אינו
רק לאחר המות אלא אף בחיי העולם הזה ההבטחה היא שנזכה לנבואה ולרוח הקודש והשפע האלוהי יושפע עלינו עד כי יעשה ה' בקרבנו נפלאות וינחילנו כבוד וגדולה, ומשום זה לא הבטיחה התורה אם תעשו מצווה זאת אנחילכם לאחר המות הנאות ואביאכם אל גנות ופרדסים אלא מה הבטיחה התורה ההבטחה היא אתם תהיו לי לעם ואני אהיה לכם לאלוהים ואהיה לכם למנהיג [וכן הבטיח ה'] שיהיו מבינינו חסידים העובדים לפני ה', הנביאים העולים בנפשם לשמים בעת נבואתם ויתהלכו בין המלאכים"
רק לאחר המות אלא אף בחיי העולם הזה ההבטחה היא שנזכה לנבואה ולרוח הקודש והשפע האלוהי יושפע עלינו עד כי יעשה ה' בקרבנו נפלאות וינחילנו כבוד וגדולה, ומשום זה לא הבטיחה התורה אם תעשו מצווה זאת אנחילכם לאחר המות הנאות ואביאכם אל גנות ופרדסים אלא מה הבטיחה התורה ההבטחה היא אתם תהיו לי לעם ואני אהיה לכם לאלוהים ואהיה לכם למנהיג [וכן הבטיח ה'] שיהיו מבינינו חסידים העובדים לפני ה', הנביאים העולים בנפשם לשמים בעת נבואתם ויתהלכו בין המלאכים"
105. The Rabbi: They are none of them [reward and punishment] realized till after death [in other religions], whilst during this life nothing points to them.
109. The Rabbi: Now all that our promises imply is that we shall become connected with the divine influence by means of prophecy, or something nearly approaching it, and also through our relation to the divine influence, as displayed to us in grand and awe-inspiring miracles. Therefore we do not find in the Bible: 'If you keep this law, I will bring you after death into beautiful gardens and great pleasures.' On the contrary it is said:'You shall be my chosen people, and I will be a God unto you, who will guide you. Whoever of you comes to me, and ascends to heaven, is as those who, themselves, dwell among the angels and my angels shall dwell among them on earth. You shall see them singly or in hosts, watching you and fighting for you without your joining in the fight. You shall remain in the country which forms a stepping-stone to this degree, viz. the Holy Land. Its fertility or barrenness, its happiness or misfortune, depend upon the divine Influence which your conduct will merit, whilst the rest of the world would continue its natural course. For if the divine presence is among you, you will perceive by the fertility of your country, by the regularity with which your rainfalls appear in their due seasons, by your victories over your enemies in spite of your inferior numbers, that your affairs are not managed by simple laws of nature, but by the divine Will. You also see that drought, death, and wild beasts pursue you as a result of disobedience, although the whole world lives in peace. This shows you that your concerns are arranged by a higher power than mere nature.' All this, the laws included, is closely connected with the promises, and no disappointment is feared. All these promises have one basis, viz. the anticipation of being near God and His hosts. He who attains this degree need not fear death, as is clearly demonstrated in our Law.
3. Knowing His Will
A third reason we need nevua is that through nevua Hashem can communicate His will.
והסיבה השלישית למציאות הנבואה היא כי ע"י הנבואה יודיעו הדברים הנרצים אצל השי"ת מהבלתי נרצים, כי הידיעה המחקרית אינה מספקת לדעת הדברים הנרצים אצלו כדי שבאמצעותם יגיע האדם להשגת השלימות האנושי.
מבי"ט בית אלהים שער היסודות פי"ב
How is it possible to know what G-d wants in any given situation? We can't just use our intellectual faculties to discern Hashem will which is supra-intellectual so for that we need a Navi.
The Rambam writes in his Moreh Nevuchim [3/45] that without nevua there is no Torah. How did Hashem communicate all of the mitzvos to Moshe if not through prophecy? Also, in the later generations we see that the prophets would ask Hashem what He wanted from us [go to war or not etc.]. See also the Drashos HaRan Drush Chamishi.
