פירוש על ברכות 52:1

If he erred and did not pray mincha on Friday etc.; He erred and did not pray mincha on shabbat he prays at arbit twice. This is to say, on Saturday night, and he says havdalah in the first, etc. Rabeinu Yehuda wrote that if he erred and did not mention Rosh Chodesh at mincha he should not pray an extra time in the evening, for what will this accomplish? He already prayed mincha, he just forgot to add [yaaleh ve'yavoh for] Rosh Chodesh. If so, he does not gain anything if he prays an extra time in the evening after Rosh Chodesh has ended, since he will not mention Rosh Chodesh [in the extra prayer], and he has already prayed Amida. And this is different than the case here, where one erred and did not pray mincha on shabbat [at all], where in the evening he prays an extra time using the Amida of a regular evening, since here he did not pray at all so at least he gains the extra amida when he prays after shabbat ends in the evening, even though he is not mentioning shabbat. And if you will ask, isn't he praying more than required since on shabbat there were only seven blessings and he will pray 18? There is no reason to worry about this, since even on shabbat it would be appropriate to pray all 18, but this was not required because of the extra effort it would require. We find, then, that he gains the full extra prayer. But when he erred and did not mention Rosh Chodesh, he has already prayed, so praying an additional time on Saturday evening will yield nothing. And similarly, if he prayed a regular weekday amida at mincha on shabbat, and did not mention shabbat, it would seem that he should not pray again Saturday evening since he already said the amida. However, Rav Alfas [the Rif] does not seem to maintain this position, and in this case even though he does not gain anything he requires an extra prayer. To quote him: "With regard to the statement 'make havdalah in the first one and not in the second one, and if you did not say havdalah in the first one but did say havdalah in the second one, the second counts but the first does not count', since he needed to say the second one first; since he did not say havdalah in the first one but said havdalah in the second one, he has shown his intention that the second one should be his normal prayer. Therefore, he must pray again another time for his make-up prayer so that the obligatory prayer precedes the make-up. And if he said havdalah in both of them, even though he was not required to do so, he does not need to repeat. And if he did not say havdalah in either of them, we do not make him repeat, as we learn in a braita 'if on erred and did not say havdalah in He Who grants us wisdom, we do not make him repeat since he can say havdalah on the cup of wine'". Therefore, we require him to pray again even though he will not gain anything additional the second time. This is what Rabbi Moshe Malavar concluded from the words of Rabbi Alfas.

See Singer, p. 46.

Because it is said only to compensate for the omission.

This Baraita confirms the opinion of Rab Huna b. Judah in the preceding paragraph, that the omitted afternoon Tefillah can be compensated for in the evening.