Why do Ashkenazim make a shin when wrapping the tefillin on the hand? The shin on the bayit of the shel rosh, the dalet on the knot of the shel rosh, and the yud on the knot of the arm tefillin already spell out the shem Hashem. What purpose does the shin on the hand serve?

ב"ה Shalom The Shin is also created according to the Chassidic custom and not only a Shin but if you look carefully enough you will notice a Dalet and Yud as well created by the finger windings. The basis for the custom of Hashem's name with the knots and the Bayit is mentioned in the Talmud (Shabbat 62.) and it is "Halacha lemoshe Misinai." Chodesh Tov and All the best