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A prophet is an advocator for the will of God, and a predictor of future events. Prophets are advocates for the Holy Spirit and serve as conduits who freely pass along knowledge of God's will. How this prophetic knowledge is passed along to others occurs through both utterances and teachings, and varies from person to person. Sometimes music is used while prophetic words are being released as well.
Prophetic knowledge which is given to those who trust in the Lord and vice-versa (which is important), often requires an interpretation by the one who initially received the gift of knowledge. From my humble opinion, it is likely ill-advised for a third party to interpret one's personal experiences with the Lord. We are all unique, and God speaks to each of us in a particular tone that we can relate to. Ultimately, the truth is what aligns with the B-I-B-L-E.
Although certain themes of prophetic knowledge might appear similar from one prophet to the next, what's dropped into one's spirit cannot be fully explained through words directed towards the human ear. 
A good portion of the Bible is prophecy, and as such - was written by ancient Israel's and Judah's prophets.
The major prophets of the Bible were Daniel, Ezekiel, Jeremiah and Isaiah of the Old Testament. These are referred to as major prophets of the Bible. This is likely due to the number of pages which were set aside for each within the Bible. If we could speak with peoples of biblical-times past, they likely had differing opinions.
Minor prophets within the Bible, include those such as Habakkuk, Joel and Zechariah.
I can't give you all the reasons why God assigns duties to his prophets, but I can certainly give you a couple of them. First of all, prophecy increases one's faith. Not only for the prophet's audience, but for the prophet as well. This brings us all closer to the Lord.
Secondly, prophets are here to warn people of upcoming-events so that they can be prepared and even change-course when required. Here, prayer is important. In the spirit realm, circumstances can often be changed and attacks by the enemy nullified.  
From my current understanding, there are certain times in history where prophets are needed more than others. Week by week now, world events are changing rather quickly. Much of biblical prophecy was written for our current-time, and the depth of modern-day prophets are here to support humanity as a whole.
 As a regular reader of the Bible, I can comfortably attest to the following. The Bible is full of knowledge, much of which includes prophetic events. Therefore, anyone who regularly reads the Bible begins to understand upcoming-events and soaks them in. Similar to osmosis, it's often just a matter of time.
Please consider the following verses as my closing remarks.
Acts 2 of the KJV, verse 17: And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams: (18) And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy:
First Corinthians 2 of the KJV, verse 9: But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.
In addition to the prior condensed-list, a great site to follow is Flyover Conservatives which is hosted by David and Stacy Whited. Periodically, this couple summarizes words from a number of God's prophets. Steve Shultz who is the founder of the Elijah List, is another who has his finger on the pulse of the prophetic.
Further, here's a link to a comprehensive prophecy index by Steven Lake. 
Rev. 251017