Dear Catholic Warrior....
Q. "I have been told that since we are absolved of our sins during the mass that one may always go to communion (if you are a Catholic in good standing of course) So I wonder what is the need for personal confession if one does not have mortal sin on their soul?"
A. Although it is true that within the Mass, if we are sorry for our sins, then our venial sins are forgiven, mortal sins can only be forgiven within the Sacrament of Reconciliation (Confession) or with a perfect act of contrition. A perfect act of contrition is where we are sorry for our sins because the sin offends God, Whom we love above all things for His own sake, and not because we fear punishment (e.g. Hell). However, even if we make a perfect act of contrition it is necessary for us to go to Confession as soon as possible so as to obey Christ by confessing our sins, and to ensure that we have been forgiven. So, mortal sins are not absolved during the Mass, and one cannot receive Holy Communion until all mortal sins have been confessed, to the best of the individual's knowledge. So let's get to your question: "what is the need for personal confession if one does not have mortal sin on their soul?"
To understand this you must know that Confession serves more than just the function of forgiving sins. Yes, that is the fundamental and most important part of the Sacrament, but it is not the only result that will occur from receiving it. When we go to Confession, not only are our sins forgiven, but we receive help to overcome those sins, to keep in a state of grace, and to grow in virtue. In a word it's called grace. Confession therefore is twofold---it forgives (helping us get up), and it helps us to sin no more (helps us to move forward in our life with Christ).
So, we go to Confession not only to receive the forgiveness that Christ offers us, but to help us to follow Christ more closely. That is one reason it is important to go to Confession frequently even if you only have "small" sins.
Here's another reason. We tend to not understand how dangerous venial sins are. Satan will generally always start small in a person's life to corrupt him. Usually you don't see children committing huge sins but rather small ones. But that is the way the big sins can enter eventually. We do a small sin enough times and eventually it becomes quite easy for us to do bigger and bigger sins, until we finally give into the mortal (deadly) sins. They are called mortal sins because we no longer have life with Christ until they are forgiven.
Perhaps the Catechism of the Catholic Church explains it best as to the value of confessing even our venial sins:
"Without being strictly necessary, confession of everyday faults (venial sins) is nevertheless strongly recommended by the Church. Indeed the regular confession of our venial sins helps us form our conscience, fight against evil tendencies, let ourselves be healed by Christ and progress in the life of the Spirit. By receiving more frequently through this sacrament the gift of the Father's mercy, we are spurred to be merciful as he is merciful:" (CCC #1458)
Hope that helped!
---The Catholic Warrior (PBXVI)