Why did Avraham build a Mizbe'ach at this point?
Rashi: To thank Hashem for the good news that, a. he would have children; and b. that they would inherit Eretz Yisrael.
Ramban #1: As an act of worship in the holy land of Cana'an. He did not do so immediately upon entering the land because he was afraid, 1 but now Hashem had allayed his fears.
Targum Yonasan and Ramban #2: To pay homage and give thanks to Hashem who had appeared to him.
QUESTIONS ON RASHI
Rashi writes: "'He built a Mizbe'ach' - [in gratitude for] the news that he would have offspring." Hadn't Hashem already assured Avraham of this when he left Charan?
Gur Aryeh: An assurance made on condition (in this case, on condition that Avraham go to Eretz Yisrael) can be rescinded before the condition is met, or be spoiled by the recipient's conduct before fulfilling the condition. This verse was an absolute assurance with no stipulations; and for this Avraham gave thanks.