Rashi
לך לך. לַהֲנָאָתְךָ וּלְטוֹבָתְךָ, שָׁם אֶעֶשְׂךָ לְגוֹי גָדוֹל, כָּאן אִי אַתָּה זוֹכֶה לְבָנִים, וְעוֹד שֶׁאוֹדִיעַ טִבְעֲךָ בָּעוֹלָם:
לך לך GET THEE OUT (literally, go for thyself): for your own benefit, for your own good: there I will make of you a great nation whilst here you will not merit the privilege of having children (Rosh Hashanah 16b). Furthermore, I shall make known your character throughout the world (Midrash Tanchuma, Lech Lecha 3)
Ramban
וַיֹּאמֶר ה' אֶל אַבְרָם לֶךְ לְךָ לַהֲנָאָתְךָ וּלְטוֹבָתְךָ, וְשָׁם אֶעֶשְׂךָ לְגוֹי גָּדוֹל, וְכָאן אִי אַתָּה זוֹכֶה לְבָנִים, לְשׁוֹן רַשִׁ"י (רש"י על בראשית י"ב:א'). וְאֵין צֹרֶךְ, כִּי מִשְׁפָּט הַלָּשׁוֹן כֵּן, "הַגֶּשֶׁם חָלַף הָלַךְ לוֹ" (שיר השירים ב יא), "אֵלֲכָה לִּי אֶל הַגְּדֹלִים" (ירמיהו ה ה), "קֻמוּ וְעִבְרוּ לָכֶם אֶת נַחַל זָרֶד" (דברים ב יג), וְרֻבָּם כָּכָה. …
AND THE ETERNAL SAID UNTO ABRAHAM ‘LECH LECHA’ (GET THEE OUT). This means, “For your own benefit and for your own good. And there I will make of you a great nation whilst here you will not merit the privilege of having children.” Thus the language of Rashi. Now there is no need for it for such is the normal expression of the Hebrew language as in the verses: The rain is over and gone ‘lo’; I will get ‘li’ unto the great men; Rise up, and get ‘lachem’ over the brook Zered; and many similar examples. …
Ramban quotes Rashi, then answers: there is no need for this.