This post is to help partially answer the same question that was answered in my last one about the Book of Mormon. I wanted to explain more in detail why we need Prophets. I think in recent years in our modern civilization, the term "Prophet" has received a bad connotation, but in ancient days, Prophets were revered.
--The Prophet receives revelation for the church as a whole. Another term for this is "Seer". We believe that individuals can receive revelation for their own lives, but you can imagine the chaos if any church member received revelation to guide the church. Each person receives revelation, or inspiration for their sphere of responsibility and authority. Just as the prophet Moses led the children of Israel, the prophet today receives revelation about what direction the church as a whole should go. "Surely the Lord God will do nothing, but he revealeth his secret unto his servants the prophets" (Amos 3:7)
--The Prophet receives scripture for the church. Once again, you can imagine the chaos if every member were allowed to add scripture. In modern day scripture we are told, "Behold, mine house is a house of order, saith the Lord God, and not a house of confusion." (Doctrine and Covenants 132:8)
--Prophets testify of Jesus Christ.
"18 But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled.
19 ¶ Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord;
20 And he shall send Jesus Christ, which before was preached unto you:
21 Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began" Acts 3:18-21
---Prophets act as "watchmen on the tower" to raise a warning voice about danger that is coming. Many times our prophets have warned us of things that would be dangerous for us. For example, Joseph Smith recieved the revelation known as the "Word of Wisdom". It counseled us not to drink alcohol, tea, or coffee, and to not use tobacco. This was given many years before it was known the dangerous effects of these substances. While tea and coffee are generally not thought to be as harmful to the body as alcohol and tobacco, still physicians will agree that we are better off without them.
The Bible dictionary had this to say about prophets: "The work of a Hebrew prophet was to act as God's messenger and make known God's will. The message was usually prefaced with the words "Thus saith Jehovah." He taught men about God's character, showing the full meaning of his dealings with Israel in the past. It was therefore part of the prophetic office to preserve and edit the records of the nation's history; and such historical books as Joshua, Judges, 1 and 2 Sam., 1 and 2 Kings were known by the Jews as the former Prophets. It was also the prophet's duty to denounce sin and foretell its punishment, and to redress, so far as he could, both public and private wrongs. He was to be, above all, a preacher of righteousness. When the people had fallen away from a true faith in Jehovah, the prophets had to try to restore that faith and remove false views about the character of God and the nature of the Divine requirement. In certain cases prophets predicted future events, e.g., there are the very important prophecies announcing the coming of Messiah's kingdom; but as a rule prophet was a forthteller rather than a foreteller. In a general sense a prophet is anyone who has a testimony of Jesus Christ by the Holy Ghost, as in Num. 11: 25-29; Rev. 19: 10." (Bible Dictionary..."prophet")