What is the special significance of prayers said in a hushed voice by the celebrant during the Holy Mass of Catholic Church?

There are certain prayers in the Catholic Holy Mass across Rites, which the celebrant says in a hushed voice, barely audible to the faithful. In the pre-Vatican II days when both the faithful and the celebrant faced the altar, such “personal prayers” would not even be heard by the faithful. But now with the celebrant facing the faithful during the liturgical celebration, and with a microphone positioned near the former, the hushed prayer becomes audible and understood by the faithful. But then, why not make the prayer in a normal voice so that the faithful can hear it? My question therefore is: **What is the special significance of prayers said in a hushed voice by the celebrant during the Holy Mass of the Catholic Church?**