Two Facets Of Tefillin

The Rambam in Laws of Tefillin (4:25) says קְדֻשַּׁת תְּפִלִּין קְדֻשָּׁתָן גְּדוֹלָה הִיא. שֶׁכָּל זְמַן שֶׁהַתְּפִלִּין בְּרֹאשׁוֹ שֶׁל אָדָם וְעַל זְרוֹעוֹ הוּא עָנָו וִירֵא שָׁמַיִם וְאֵינוֹ נִמְשָׁךְ בִּשְׂחוֹק וּבְשִׂיחָה בְּטֵלָה וְאֵינוֹ מְהַרְהֵר מַחֲשָׁבוֹת רָעוֹת אֶלָּא מְפַנֶּה לִבּוֹ בְּדִבְרֵי הָאֱמֶת וְהַצֶּדֶק. לְפִיכָךְ צָרִיךְ אָדָם לְהִשְׁתַּדֵּל לִהְיוֹתָן עָלָיו כָּל הַיּוֹם שֶׁמִּצְוָתָן כָּךְ הִיא. אָמְרוּ עָלָיו עַל רַב תַּלְמִידוֹ שֶׁל רַבֵּנוּ הַקָּדוֹשׁ שֶׁכָּל יָמָיו לֹא רָאוּהוּ שֶׁהָלַךְ אַרְבַּע אַמּוֹת בְּלֹא תּוֹרָה אוֹ בְּלֹא צִיצִית אוֹ בְּלֹא תְּפִלִּין:  The Rambam seems to be giving a reason why one should wear tefillin all day long.  Why is any explanation necessary; that is the mitzvah?  There are mitzvot that one does in one action like taking a lulav, blowing shofar etc. but then there are mitzvot that are constant like the mitzvah of loving and fearing G-d.  The mitzvah of tefillin is in the second category; it applies all day, so why does the Rambam need to add a reason that tefillin on a person leads to יראת שמים?  Rav Solovetchik explains that there are two aspects to the mitzvah of tefillin.  There is the fulfillment of placing tefillin of one's body.  There is a second fulfillment to the mitzvah and that is the person becomes sanctified by the tefillin being on him.  The idea of this second aspect is understood from the possuk in Ki Savo (28:10)  וראו כל עמי הארץ כי שם ה' נקרא עליך ויראו ממך and the Gemorah Menachos (35b) explains תניא ר"א הגדול אומר אלו תפילין שבראש.  The tefillin affect a change in the person that is recognizable even to outsiders that there are תפילין שבראש, in the person.  These two fulfillment's are reflected in the two berachos that one says on the tefillin according to Rabbenu Tam.  The first beracha, להניח is on the fulfillment of placing tefillin on one's arm.  The second beracha, כל מצות תפילין which goes on the head tefillin is another beracha for the additional fulfillment of adorning the individual with tefillin.
With this idea, he explains the aforementioned Rambam.  The Rambam held had tefillin been a mitzvah just to put tefillin on one's body then the mitzvah would be just to to the action once a day.  It is the second aspect of the mitzvah, the fulfillment in the person that makes it a mitzvah to be adorned all day long.  That is the explanation of the reason the Rambam gives for it being a all day mitzvah.  The Rambam is describing the affect tefillin has on the individual and that's what transforms the mitzvah into a all day mitzvah (שעורים לזכר אבא מרי volume 1 pg. 180-181 בענין קריה"ת בשבת שני וחמישי.)