The Rema Orach Chaim Siman 682 writes,
הגה י"א כששכח על הנסים בברכת המזון כשמגיע להרחמן יאמר הרחמן יעשה לנו נסים ונפלאות כשם שעשית לאבותינו בימים ההם בזמן הזה בימי מתתיהו כו' [כל בו]
Gloss: There are opinions that if one forget "For the miracles" in the grace after meals, when one reaches "the merciful one", say: "May the merciful one make miracles for us just like You make for our forefathers in those days at this time, in the days of Matityahu etc. (Sefaria translation)
Alot was written regarding this blessing of הרחמן, becaues it seems to contradict what the Gemara says in Berachos 60a that one may not pray for miracles (see here שערי תשובה או"ח סי' קפ"ז https://www.sefaria.org/Sha'arei_Teshuvah_on_Shulchan_Arukh%2C_Orach_Chayim.187.4?lang=bi&with=all&lang2=en).
Many ideas were discussed throughout the generations, but the common denominator is that at that point in time (either Chanukah or Purim) one does have an unique opportunity to pray for miracles...
Since this הרחמן is such a unique and special prayer (praying for miracles), then why does one merit to say it 'only' when he forgot to say the 'real thing' (על הניסים)?