Remember Avraham, Yitzchak and Yisrael Your servants, that You swore to them by Your very Self, and You told them, "I shall increase your offspring like the stars of the heavens, and the entire land of which I spoke, I shall give to your offspring and they shall have it as a heritage forever".
After the Jews sinned with the Golden Calf, Hashem told Moshe, "Let My anger burn against them and I shall annihilate them". Moshe then prayed on his Nation's behalf, and beseeched Hashem to remember that which He swore to the forefathers, that 'He will increase their offspring like the stars of the heavens, and that He will give the entire land of which He spoke to their offspring".
We can understand why Moshe would plead that Hashem remember that which He swore 'that He will increase their offspring like the stars of the heavens', for that itself was a reason why Hashem should forgive the Jews and not annihilate them. But what benefit was there for Hashem to remember that He swore to give the entire land to the Jews? On the contrary, their sin itself was a reason that they should no longer be worthy of receiving the Land.
The Mishnah in Kiddushin
The Mishnah in Kiddushin (ס ע"א) teaches us the following Halacha.
האומר לאשה הרי את מקודשת לי על מנת שאתן לך מאתים זוז הרי זו מקודשת והוא יתן
If one says to a women, "You are betrothed to me on condition that I will give you two hundred zuz", the law is that this women is betrothed to him, and he shall give it to her. [The Gemara goes on to discuss the meaning of these last words, 'and he shall give it to her'.]
איתמר רב הונא אמר והוא יתן רב יהודה אמר לכשיתן וכו', מאי בינייהו איכא בינייהו שפשטה ידה וקבלה קידושין מאחר, לרב הונא לא הוו קידושי לרב יהודה הוו קידושי
Rav Huna said, this means that he should give her the money whenever he wishes. Once he pays her, the betrothal takes effect retroactively from the time that he performed the marriage act. But Rav Yehudah said, it means that when he gives her the money, the betrothal takes effect; it does not take effect retroactively... What is the practical difference between these two opinions? The difference between them is with regard to a case in which the woman extended her hand and accepted a betrothal proposal from another man before the original suitor gave her the two hundred zuz. According to Rav Huna, the betrothal of the second man is not valid, while according to Rav Yehudah, it is valid. [i.e. According to Rav Huna, the first man's betrothal takes effect retroactively, preempting the betrothal of the second man. According to Rav Yehudah, however, the first man's betrothal goes into effect only after he actually gives the two hundred zuz, and thus, since the second man betrothed the women before this money was given, the second man's betrothal is valid.]
When Hashem gave the Torah to the Jewish Nation, it was, so to speak, an act of betrothal, through which the Jewish People were bound to Hashem and his Torah. Correspondingly, when Hashem told our forefathers that He would give the Land to their offspring, it was as if He betrothed the Jewish Nation on condition that He would give them that Land.
Accordingly, we can explain why Moshe found it important to mention Hashem's promise to give the Land to the Jewish People when beseeching Hashem to forgive the Jews for the sin of the golden calf. Because the giving of the Land to the Jewish people was a condition upon which their betrothal and bond to the Torah was dependent, therefore, at least according to Rav Yehudah, so long that Hashem did not give them the Land, they were essentially not yet 'betrothed' to Him. Hence, the Jewish People were not yet committed to the Torah, and thus Hashem could not hold their sins against them, and could not be angry at them.
And even according to Rav Huna, whose opinion is that when the Land would eventually be given to them the betrothal would be effective retroactively, and as a result the Jewish people were indeed bound to the Torah and obligated to keep its laws even before receiving the Land, nevertheless, Moshe claimed that because Hashem swore to our forefathers to give the Land to the Jewish People, He was still obligated to do so even after they sinned. Accordingly, Moshe asserted, once they would enter the Holy Land that in itself would suffice to bring upon them forgiveness, as the Passuk in Parshas Haazinu (ל"ב כ"ג) says, וכפר אדמתו עמו - and His land will atone for His people, as well as the Passuk in Yeshaya (ל"ג כ"ד) says, העם היושב בה נשוא עון - The people dwelling there shall be forgiven of sin.
('זרע שמשון' פרשתנו אות יח)
