Genesis 41:45 JPS
And Pharaoh called Joseph's name Zaphenath-paneah; and he gave him to wife Asenath the daughter of Poti-phera priest of On. And Joseph went out over the land of Egypt.
וַיִּקְרָא פַרְעֹה שֵׁם-יוֹסֵף, צָפְנַת פַּעְנֵחַ, וַיִּתֶּן-לוֹ אֶת-אָסְנַת בַּת-פּוֹטִי פֶרַע כֹּהֵן אֹן, לְאִשָּׁה; וַיֵּצֵא יוֹסֵף, עַל-אֶרֶץ מִצְרָיִם.
Genesis 41:50 JPS
And unto Joseph were born two sons before the year of famine came, whom Asenath the daughter of Poti-phera priest of On bore unto him.
וּלְיוֹסֵף יֻלַּד שְׁנֵי בָנִים, בְּטֶרֶם תָּבוֹא שְׁנַת הָרָעָב, אֲשֶׁר יָלְדָה-לּוֹ אָסְנַת, בַּת-פּוֹטִי פֶרַע כֹּהֵן אוֹן.
Genesis 46:20 JPS
And unto Joseph in the land of Egypt were born Manasseh and Ephraim, whom Asenath the daughter of Poti-phera priest of On bore unto him.
וַיִּוָּלֵד לְיוֹסֵף, בְּאֶרֶץ מִצְרַיִם, אֲשֶׁר יָלְדָה-לּוֹ אָסְנַת, בַּת-פּוֹטִי פֶרַע כֹּהֵן אֹן--אֶת-מְנַשֶּׁה, וְאֶת-אֶפְרָיִם.
Why is it so important to know that Poti-phera is a priest of On? Genesis appears to be making a point here.