As a recently ordained, I went to the Vatican and was able to pray a Mass in St. Peter. I was assigned a little altar server who grabbed a Mass kit for me and led me through the cavernous space to some chapel altar. There were priests and altar boys all over the place saying Masses. Here and there were lay people, who woke at an hour I think God did not intend for us to be awake, who were in attendance at various altars.
The server assisted me in finding my altar, helped set up and get Mass going. It was wonderful not to have to figure things out on my own with a space in which I was rather unfamiliar.
One would think Mass is Mass is Mass. But in every parish things are done just a little bit differently. How and when are the gifts brought up? (Before the collection? After the collection? When the altar is set up?) When does the Gospel Alleluia begin? (When the priest stands? When the deacon stands? When the organist is good and ready?) How is Communion distributed? These may seem like little things but they are terribly distracting when one is attempting to pay attention to the prayers.
It is always a great thing to have someone who knows what they are doing guide you around. At St. Sebastian we attempt to try to have someone on with a visiting priest that can help them navigate the way things are done here. With a visiting server there is an effort to have an experienced server on with him to make it less stressful.
These are some of the reasons paragraph 106 of the GIRM says that it is desirable in cathedrals and in larger churches (though it might be helpful just about anywhere) “to have a competent minster or master of ceremonies, to see to the appropriate arrangement of sacred actions and to their being carried out by the sacred minsters and lay faithful with decorum, order and devotion.” In other words, allowing the ministers not to become the center of attention as they turn, look around, and in general look helpless wandering around the sanctuary.