Rav Avraham Hakohein, the grandson of the Matnos Kehunah, explained that the term of Chazal (Brachos 53b) “gadol ha’oneh amen yoser min hamevarech” includes also the actual reason for why the virtue of the oneh amen is greater than the virtue of the mevarech. Because the word “yoser” seems to be superfluous here, and it would have been enough to say, “gadol ha’oneh min hamevarech.” But this is explained as follows: “Gadol ha’oneh amen” because he could can answer “yoser min hamevarech.” In contrast to the mevarech, who cannot make brachos as he pleases, and only can make the brachos he is obligated to say, because he is not allowed to make a brachah she’einah tzrichah, one who answers amen can fulfill the mitzvah of answering amen as much as he wants, because he can hear the brachos from many mevarchim, and answer amen after them. This is the virtue of the oneh, because he can answer over and over “yoser min hamevarech.” (Ori Veyishi [Berlin 5474] Sha’ar Hatzedakah Perek Shemini)
