The Alter Rebbe teaches that the purpose of the soul's descent into this world is to make a dwelling place for God through Torah and mitzvot, especially when material life is transformed toward holiness. Individual purpose is thus expressed through one's particular opportunities for such transformation.
כִּי כְּלָלוּת יִשְׂרָאֵל, שֶׁהֵם שִׁשִּׁים רִבּוֹא נְשָׁמוֹת פְּרָטִיּוֹת, הֵם כְּלָלוּת הַחַיּוּת שֶׁל כְּלָלוּת הָעוֹלָם, כִּי בִּשְׁבִילָם נִבְרָא, וְכָל פְּרָט מֵהֶם הוּא כּוֹלֵל וְשַׁיָּיךְ לוֹ הַחַיּוּת שֶׁל חֵלֶק אֶחָד מִשִּׁשִּׁים רִבּוֹא מִכְּלָלוּת הָעוֹלָם הַתָּלוּי בְּנַפְשׁוֹ הַחִיּוּנִית, לְהַעֲלוֹתוֹ לַה׳ בַּעֲלִיָּיתָהּ, דְּהַיְינוּ, בְּמַה שֶּׁמִּשְׁתַּמֵּשׁ מֵעוֹלָם הַזֶּה לְצוֹרֶךְ גּוּפוֹ וְנַפְשׁוֹ הַחִיּוּנִית לַעֲבוֹדַת ה׳, כְּגוֹן, אֲכִילָה וּשְׁתִיָּה וְדוֹמֵיהֶם, וְדִירָה וְכָל כְּלֵי תַּשְׁמִישָׁיו. וְכָל נִיצוֹץ, לֹא יָרַד לָעוֹלָם הַזֶּה, אַף שֶׁהִיא יְרִידָה גְדוֹלָה וּבְחִינַת גָּלוּת מַמָּשׁ, כִּי גַם שֶׁיִּהְיֶה צַדִּיק גָּמוּר עוֹבֵד ה׳ בְּיִרְאָה וְאַהֲבָה רַבָּה בְּתַעֲנוּגִים לֹא יַגִּיעַ לְמַעֲלוֹת דְּבֵיקוּתוֹ בַּה׳ בִּדְחִילוּ וּרְחִימוּ בְּטֶרֶם יְרִידָתוֹ לָעוֹלָם הַזֶּה הַחוּמְרִי, לֹא מִינָּהּ וְלֹא מִקְצָתָהּ, וְאֵין עֵרֶךְ וְדִמְיוֹן בֵּינֵיהֶם כְּלָל, כַּנּוֹדָע לְכָל מַשְׂכִּיל, שֶׁהַגּוּף אֵינוֹ יָכוֹל לִסְבּוֹל כוּ׳, אֶלָּא יְרִידָתוֹ לָעוֹלָם הַזֶּה לְהִתְלַבֵּשׁ בַּגּוּף וְנֶפֶשׁ הַחִיּוּנִית, הוּא כְּדֵי לְתַקְּנָם בִּלְבָד, וּלְהַפְרִידָם מֵהָרָע שֶׁל שָׁלשׁ קְלִיפּוֹת הַטְּמֵאוֹת, עַל יְדֵי שְׁמִירַת שַׁסַ״ה לֹא תַעֲשֶׂה וְעַנְפֵיהֶן, וּלְהַעֲלוֹת נַפְשׁוֹ הַחִיּוּנִית עִם חֶלְקָהּ הַשַּׁיָּיךְ לָהּ מִכְּלָלוּת עוֹלָם הַזֶּה, וּלְקַשְּׁרָם וּלְיַיחֲדָם בְּאוֹר־אֵין־סוֹף בָּרוּךְ־הוּא, אֲשֶׁר יַמְשִׁיךְ בָּהֶם עַל יְדֵי קִיּוּמוֹ כָּל רַמַ״ח מִצְוֹת עֲשֵׂה בְּנַפְשׁוֹ הַחִיּוּנִית, שֶׁהִיא הִיא הַמְקַיֶּימֶת כָּל מִצְוֹת מַעֲשִׂיּוֹת כַּנִּזְכָּר לְעֵיל.
For the community of Israel, comprising 600,000 particular souls, is the [source of] life for the world as a whole, which was created for their sake. And each one of them contains and is related to the vitality of one part in 600,000 of the totality of the world, which [part] depends on his vital soul for its elevation to G–d through its own [the soul’s] elevation, by virtue of the individual’s partaking of this world for the needs of his body and vital soul in the service of G–d, viz., eating, drinking, and the like, [his] dwelling and all his utensils. And each spark descended into this world—although it is indeed a profound descent and a state of true exile, for even if one be a perfectly righteous person, serving G–d with fear and a great love of delights, he cannot attain to the degree of attachment to G–d, in fear and love, as before it came down to this gross world, not a fraction of it, and there is no comparison or similarity between them at all, as is clear to every intelligent person, for the body cannot endure, and so on, nevertheless [each spark] descended into this world, to be clothed in a body and vital soul, for the sole purpose of mending them and separating them from the evil of the three impure kelipot, through the observance of the 365 prohibitions and their offshoots, and in order to elevate his vital soul together with its portion that belongs to it of the totality of the world, so as to join and unite them with the light of the En Sof, blessed is He, which the person draws into them through fulfilling all the 248 positive precepts through the agency of the vital soul, the very one that fulfills all the active commandments, as has been explained above.