In 13:3,9,14,15,17, etc. the passuk says "For with a mighty hand I brought you out of Egypt".
Why keep repeating so many times that He took us out with a mighty hand? we got the point already! And why is it so relevant? The Torah is giving us a few mitzvos, why not just say "remember I took you out of Egypt, why the "mighty hand" so many times?
Strength is obviously an important facet of rule and leadership. Strength is deterrence. That strength is what kept most of the hostile nations away from attacking the Jews in the desert. the strength had to be stressed to keep everyone else at bay.
Strength is important, and not just to the outside. It is also important internally. People rally around and follow a strong leader. In the psukim here it is being used internally. It is being used to explain the reason for the mitzvos being given here; tefillin, redeeming the first-born, Pesach. We need to see the strength of our leader and that gives us what to follow. Nobody wants to follow a weak leader. By knowing that we put on tefillin because of the strong hand, it gives us reason to continue putting on tefillin.