If I’m understanding the question correctly, it is why we have options in the penitential rite and the eucharistic prayer.
To understand why the Sacred Congregation on Worship and Sacraments allows the choices one must understand the theology of each.
The penitential rite belongs to the deacon and the people. The deacon is the ordinary presider at the penitential rite. If there is no deacon, then it falls on the priest who is presiding, not on a concelebrant.
The deacon chooses the penitential rite according to the pastoral need. This can include the season of the year, such as someone mentioned that in certain seasons there is no gloria, therefore the Kyrie is a better version, because it lends itself to music. That’s just one pastoral reason, but it’s not the only one. Whether the mass is a funeral, wedding, religious profession, ordination or there are baptisms during the mass, or whether it’s a solemnity or a weekday with no particular liturgical significance. Also, the readings of the day are to be considered when choosing the penitential rite. Since all of these factors vary, none are stable, the Church allows for diverse posibilities that can enhance the celebration for the occassion.
The Eucharistic prayer belongs to the priest. It is meant for the priest, not the congregation, not even the deacon. The faithful are not praying at that time. It is the priest, united to the priesthood of Jesus Christ who offers the sacrifice. This is his prayer. It is a priestly prayer.
There are as many differences in priests as there are pastoral differences, as we pointed above. Some differences among priests include their spirituality, their culture, the liturgy that they’re celebrating that day, whether they belong to a religious community or are secular men, whether they feel that the congregation will be able to follow one prayer better than another. In other words, it’s the judgment of the priest to choose a Eucharistic prayer for the mass at which he presides. Hopefully, the choices will also reduce the “routine effect”. When you say the same words every day it is easy to reach a point where you are not praying, but reciting by rote. This was one of the problems that was actually addressed when the Ordinary Form was discussed. The canon often became a rote exercise for many priests. They had it memorized and they flew right through it. Hopefully, by giving the priest some choices he can discipline himself to toggle between the Eucharistic prayers to avoid falling into the “routine effect.”
Br. JR, OSF