Intercessory prayer is "standing in the gap" or bringing others to God with your prayers. Anyone can and should intercede for others, especially those who are in trouble spiritually and cannot or will not pray for themselves. Everyone, as christians, are called to pray for their brothers and sisters in this world.
There is also the charism of intercessory prayer. The Holy Spirit gives the gift of powerful, intense prayer to the intercessor. It is a burning prayer, forceful, and impossible to ignore. Well...a person could ignore it I guess, but why would they want to? The Holy Spirit will bring the person that He is calling you to pray for to your heart and mind along with the overwhelming urge to pray for this person. It seems as if you are before the Mercy Seat and the Lord shares with you His love and mercy for the one you are praying for; it may often bring you to tears, as the Lord's mercy is a thing of incredible beauty. It is a deep all-encompassing prayer, it feels as if it is being pulled out of you. You are left feeling awestruck at the depths of God's mercy. This is completely the Holy Spirit's work; the person praying is simply the vehicle as a member of the body of Christ. This charism, as with any, grows stronger when the person cooperates with the Spirit.
Oddly enough, God may bring total strangers to you before drawing you into this prayer. The Holy Spirit moves the person's heart to open up to you and they will often pour out their life story even though they do not know you. It's actually kind of strange when this happens, as it is unexpected. Other times you may be just praying in general and the Holy Spirit draws you into the intercessory prayer. Sometimes it is out of the blue, but it is enough to understand that He is doing it for a good purpose and it can help others immensely.
One of the reasons that I think the charisms are important is because they do help build up the Body of Christ on earth. You can't take charisms with you when you die, but you can sure help others by accepting these gifts, that is their purpose.