We see in Yoma 1:3 that the Kohen Gadol was read the order of Yom Kippur from the Torah as preparation during the seven days before Yom Kippur.
מָסְרוּ לוֹ זְקֵנִים מִזִּקְנֵי בֵית דִּין, וְקוֹרִין לְפָנָיו בְּסֵדֶר הַיּוֹם, וְאוֹמְרִים לוֹ, אִישִׁי כֹהֵן גָּדוֹל, קְרָא אַתָּה בְּפִיךָ, שֶׁמָּא שָׁכַחְתָּ אוֹ שֶׁמָּא לֹא לָמָדְתָּ. עֶרֶב יוֹם הַכִּפּוּרִים שַׁחֲרִית, מַעֲמִידִין אוֹתוֹ בְּשַׁעַר מִזְרָח, וּמַעֲבִירִין לְפָנָיו פָּרִים וְאֵילִים וּכְבָשִׂים, כְּדֵי שֶׁיְּהֵא מַכִּיר וְרָגִיל בָּעֲבוֹדָה:
They delivered to him elders from the elders of the court and they read before him [throughout the seven days] from the order of the day. And they say to him, “Sir, high priest, you read it yourself with your own mouth, lest you have forgotten or lest you have never learned.” On the eve of Yom HaKippurim in the morning they place him at the eastern gate and pass before him oxen, rams and sheep, so that he may recognize and become familiar with the service.
Yachin explains that the Kohen Gadol should read himself lest he had forgotten, as speaking the words oneself cements the material in memory:
קרא אתה בפיך דחיים הם למוצאיהם לזכרם עי"ז יפה [כעירובין דנ"ד א'] דשמא שכחת
But what reason is there for him to say the words himself if he had never learned this portion of the Torah before? He had just heard it from the elders of the court. Is it also to make sure that the learning is cemented in his memory?