The Chinukh on the mitzvah of reciting Shema frames the commandment as accepting God's kingship and unity, a conceptual backdrop for why 'El Melech Ne'eman' is meaningful in Shema practice.
מִשָּׁרְשֵׁי הַמִּצְוָה. שֶׁרָצָה הַשֵּׁם לְזַכּוֹת עִמּוֹ שֶׁיְּקַבְּלוּ עֲלֵיהֶם מַלְכוּתוֹ וְיִחוּדוֹ בְּכָל יוֹם וָלַיְלָה כָּל הַיָּמִים שֶׁהֵם חַיִּים, כִּי בִּהְיוֹת הָאָדָם בַּעַל חֹמֶר, נִפְתֶּה אַחַר הַבְלֵי הָעוֹלָם וְנִמְשָׁךְ לְתַאֲווֹתָיו צָרִיךְ עַל כָּל פָּנִים זִכָּרוֹן תְּמִידִי בְּמַלְכוּת שָׁמַיִם לְשָׁמְרוֹ מִן הַחֵטְא, עַל כֵּן הָיָה מֵחַסְדּוֹ לְזַכּוֹתֵנוּ וְצִוָּנוּ לְזָכְרוֹ שְׁנֵי הָעִתִּים הָאֵלֶּה בְּקֶבַע וּבְכַוָּנָה גְּמוּרָה, אַחַת בַּיּוֹם לְהוֹעִיל לְכָל מַעֲשֵׂינוּ שֶׁבַּיּוֹם, כִּי בִּהְיוֹת הָאָדָם זוֹכֵר בַּבֹּקֶר אַחְדוּת הַשֵּׁם וּמַלְכוּתוֹ, וְכִי הַשְׁגָּחָתוֹ וִיכָלְתּוֹ עַל הַכֹּל, וְיִתֵּן אֶל לִבּוֹ כִּי עֵינָיו פְּקוּחוֹת עַל כָּל דְּרָכָיו, וְכָל צְעָדָיו יִסְפֹּר, לֹא יִתְעַלֵּם מִמֶּנּוּ דָּבָר מִכָּל דְּבָרָיו, וְלֹא יוּכַל מִמֶּנּוּ לְהַחְבִּיא אַחַת מִכָּל מַחְשְׁבוֹתָיו הֲלֹא יִהְיֶה לוֹ לְמִשְׁמָר מַחְשַׁבְתּוֹ זֹאת וְהוֹדָאַת פִּיו בַּדָּבָר הַזֶּה כָּל הַיּוֹם הַהוּא, וְיִהְיֶה לוֹ הוֹדָאַת הַלַּיְלָה בָּזֶה גַּם כֵּן לְמִשְׁמָר כָּל הַלַּיְלָה. וּמִפְּנֵי שֶׁיְּסוֹד הַמִּצְוָה מָה שֶׁזָּכַרְנוּ חִיְּבוּנוּ זִכְרוֹנָם לִבְרָכָה בָּהּ בְּכַוָּנַת הַלֵּב, וְאָמְרוּ שֶׁאִם לֹא כִּוֵּן לִבּוֹ בָּהּ לֹא יָצָא יְדֵי חוֹבָתוֹ, שֶׁאֵין אָדָם נִזְכָּר בְּשׁוּם דָּבָר אֶלָּא אִם כֵּן יָשִׂים כַּוָּנָתוֹ בּוֹ. וְזֶהוּ שֶׁאָמְרוּ זִכְרוֹנָם לִבְרָכָה בִּבְרָכוֹת פֶּרֶק הָיָה קוֹרֵא (ברכות יג ב) תָּנוּ רַבָּנַן שְׁמַע יִשְׂרָאֵל יְיָ אֱלֹהֵינוּ יְיָ אֶחָד, עַד כָּאן צְרִיכָה כַּוָּנַת הַלֵּב. וְהִצְרִיכוּנוּ לְהַאֲרִיךְ בְּאֶחָד, וְכִדְתַנְיָא הָתָם סוּמְכוֹס בֶּן יוֹסֵף אוֹמֵר כָּל הַמַּאֲרִיךְ בְּאֶחָד מַאֲרִיכִין לוֹ יָמָיו וּשְׁנוֹתָיו, וְאָמַר רַב אַחָא בַּר יַעֲקֹב וּבַדָּלֶ"ת, אָמַר רַב אָשֵׁי וּבִלְבַד שֶׁלֹּא יַחֲטֹף בַּחֵי"ת. וְאָמְרוּ שָׁם עַד כַּמָּה יִהְיֶה אֲרִיכוּת זֶה? וְהָיְתָה הַתְּשׁוּבָה עַד כְּדֵי שֶׁתַּמְלִיכֵהוּ בַּשָּׁמַיִם וּבָאָרֶץ וּבְאַרְבַּע רוּחוֹת הָעוֹלָם, כְּלוֹמַר, שֶׁתְּכַוֵּן שֶׁמֶּמְשַׁלְתּוֹ בַּכֹּל הִיא, וְאֵין כָּל דָּבָר נֶעְלָם מִמֶּנּוּ, וּבְחֶפְצוֹ קִיּוּם כָּל הַדְּבָרִים.
It is from the roots of the commandment that God wanted to give merit to His people that they should accept upon them His Kingdom and His unity every day and night, all the days that they are alive. As since man is a physical being [that] is seduced by the vanities of the world and drawn by his desires, he certainly requires a constant reminder of the Kingdom of the Heavens to guard him from sin. Hence it was from His kindness to have us merit and He commanded us to remember [this at] these two times, regularly and with great intention. [The] one at the day is to help for all of our actions during the day; as when a man remembers in the morning the unity of God and His Kingdom, and that His Providence and Omnipotence is over everything, and he places to his heart that His eyes are open upon all of his ways and that all of his steps are marked — that he cannot obscure anything from Him and he cannot hide any of his thoughts — will this thought and the verbal acknowledgement of this matter not be a protection for him that whole day? And the acknowledgement of the night will also be a protection the whole night. And because the foundation of the commandment is what we mentioned, they, may their memory be blessed, obligated us about intention of the heart and said that if he did not have intention in his heart, he did not fulfill his obligation. As a man does not remember anything unless he places his intention to it. And this is what they, may their memory be blessed said in Berakhot 13b in the chapter [entitled] Haya Koreh, “The Rabbis learned, ‘“Listen Israel, the Lord is our God, the Lord is one” — until here, intention of the heart is required.’” And they required us to lengthen [the word,] “one.” And [it is] like it was learned over there, “Somchos ben Yosef says, ‘Anyone who lengthens [the word,] “one,” his days and years are lengthened.’ And Rav Acha bar Yaakov said, ‘But in the [letter] dalet.’ Rav Ashi said, ‘And so long as he does not cut short the [letter] chet.’” And they said there, “How much is this lengthening?” And the answer was until you crown Him in the heavens and in the earth and in the four directions of the world; meaning to say that you have intention that His governance is over all, and that there is nothing obscured from Him and that the existence of all things is through His will.