The reforms of the Roman Liturgy in general (which includes the celebration of the Mass, the Sacraments, and the Liturgy of the Hours) that took place under Pope Paul VI have a much more complicated history and explanation than the majority of folks want to admit. It really was the crowning point of a movement that began way back in the mid-19th Century with folks like Dom Prosper Guaranger. But at the risk of being over-simplistic I’ll attempt an answer at your honest question:
One intention in the reforms of the Mass itself was to restore to the Roman Church a “purer form” of the Roman Mass. Incidentally, this was also the intention of the Council of Trent with the various reforms of the Mass mandated there. As any liturgical historian would admit, liturgy (especially at the popular level) always has a tendency to acquire new elements over time. Some of those elements can be good, others not so much; and some of those elements can be completely foreign to the liturgical understanding of the particular Church or the Church as a whole. One example would be the so-called “private Mass” that used to be celebrated prior to the current reforms. The idea of a “private Mass” is actually at odds with the intention and purpose of Liturgy to begin with. Liturgy, by definition, means “work of the people” or “a public service.” Within the context of the Church, Liturgy is the work of the Church as a whole, not the exclusive work of the priest that is merely observed by the rest of us. This is one of the reasons that the Church saw fit to do away with “private Masses” and to begin encouraging the “active/actual participation” of the lay faithful. We all have a hand in celebrating the Liturgy. As in any good drama, each character has his role to play and the action itself is not properly executed if each person isn’t playing his role.
Another intention was to restore certain practices of the ancient Church that had simply fallen out of use, but that the Church deemed appropriate for restoration according to our own peculiar times and culture. Two examples (both somewhat controversial) come to mind: 1) Communion under both kinds (i.e. the Body/bread
and the Blood/wine), 2) Communion in the hands. The first may not come off as so controversial, but there really are folks out there who believe that lay people should not be receiving the Precious Blood. Their argument goes something along the lines of “You receive the whole Christ when you receive just the Body.” This is entirely true, but the Church’s reason for restoring the practice of “Communion under both kinds” wasn’t so that the faithful, after hundreds of years of oppression (I mean this humorously), could finally receive both the Body
and the Blood. Rather, the intention was to restore a fuller symbolism in the practice of reception of Communion.
One could make a similar argument for Communion in the hands, but that has been argued ad nauseum on these forums. Suffice it to say that scholars - men and women who have spent
decades studying the sources of the early Church and Her Liturgy in the original languages - have demonstrated fairly definitively that receiving Communion in the hand was actually the universal practice throughout the Church, East and West, up until the 8th Century at the earliest, and in many areas even right through the Middle Ages. I refer you to Fr. Robert Taft’s excellent and encyclopedic
A History of the Liturgy of St. John Chrysostom, Vol. 6: The Communion, Thanksgiving, and Concluding Rites in which he demonstrates this in the context of the Eastern/Byzantine Liturgy, but references the sources that have demonstrated the same for the Roman Mass.
So those are just two of the major reasons that I’ve found for the reforms that took place in the Roman Mass and Liturgical life as a whole under Pope Paul VI. Again, I’m afraid here of being over-simplistic, but this is a good start for the sake of discussion.
