We serve our creator with theory. You shall teach them to your children to speak them while you sit in your home, whil you walk in the way, when you retire and when you arise, (Deut. 11:19)
We serve our creator with practice. Now what does Hashem your G-d ask of you? To fear Hashem your G-d, to go in all His ways and to love Him, to observe the commandments of the L-rd and His decrees (Deut. 10:12)
Both of these are necessary for our worship to be complete. We therefore place Tefillin on our head directed to our intellect and on our arm directed to our deeds to subjugate both of these to service ourr Creator.