רמב"ן — והקמתי את השבועה אשר נשבעתי לאברהם אביך

Q — Why would Hashem need to reassure Yitzchak that He'll keep the oath sworn to Avraham? Hashem 'לא אדם הוא להנחם', Avraham has no other zera-bal-brit (Yishmael was ben ha'ama), and the original commitment was a shevua, not conditional. Reassurance seems superfluous.

A — Ramban: 'והקמתי את השבועה' is not a reassurance but itself constitutes a new shevua — to Yitzchak. This explains why later Torah passages routinely cite an oath sworn to Yitzchak specifically ('הארץ אשר נשבעתי לאברהם ליצחק וליעקב') — there's no other source for it but here. Hashem made each av his own בעל ברית by swearing separately, honoring each (כל אחד 'ראוי לכרות עמו ברית') and creating distinct zechut-streams flowing to their descendants — 'וזכרתי את בריתי יעקב ואף את בריתי יצחק ואף את בריתי אברהם'. (Alternative reading: the shevua refers to 'והתברכו בזרעך' being fulfilled in Yitzchak personally, not in zera alone.)