What is the significance of the phrase, "From these, the islands of the nations were separated in their lands," which the Torah writes only regarding the sons of Yefes?
Ramban: This is because, as opposed to the sons of Cham and of Shem respectively, who lived close to one another, 1 the sons of Yefes were spread out to far-flung islands, each living in his own location. 2
This is why the Torah later describes the territory of Kena'an (son of Cham) as one border, "And the border of the Kena'ani was from Tzidon..." (10:19) (those cities were all close to each other); and that of Cham and Shem as, "in their lands, in their nations" (10: 20,31) (implying on land, but not islands in the sea).
This was due to Noach's blessing, "HaSh-m will widen the borders of Yefes" (9:27).
Why does the Torah see fit to insert all the detail about where the sons of Noach lived?
Ramban #1: To teach us the Yichus of Avraham who descended from Shem, and that of Cham, whose land the sons of Avraham were destined to inherit due to Cham's sins. (It includes the Yichus of Yefes to complete the picture.) It then tells us briefly about the Dor ha'Palagah, to teach us the source of the world's many languages, and how it came about that the nations spread across the four corners of the earth so soon after the Flood.
Ramban #2: To teach us the kindness of HaSh-m, and how He kept His oath never again to destroy the world (even they deserved to be wiped out).
Ramban #3 (citing the Rambam in Moreh Nevuchim): The current Parshiyos support our belief in the Creation; inasmuch as Avraham would pass on the testimony of; (a) the Flood, which he heard directly from Noach, 1 and (b) the Creation, about which he heard from Noach, who heard it from his father, who heard it from Adam. 2