When we put on the tefillin shel yad, I've noticed that virtually everyone ties the tefillin up until the actual hand (like a placeholder), puts on the shel rosh, and only then finally completes the tying of the shel yad.
Why is this so? Is it merely to make the tying of the shel rosh easier (by having the left [right] hand available) or is there some deeper reasoning?
This is a case where the universal custom is according to the Arizal.
In Halacha, the Mitzvah of Tefillin shel yad is to bind it to the bicep and then wrap around the fingers three times. The three fingers do not invalidate the binding though. Therefore, according to halacha, one would put the Yad and immediately put the Rosh and only afterwards wrap around the finger.
The Arizal on the other hand holds that the seven circuits around the arm is an intrinsic part of the Mitzvah. He holds this for Kabbalistic reasons. Therefore, according to him, you must complete the wrapping of the 7 circuits before putting on the Rosh.
The universal custom is according to the Arizal.
An interesting question is if the custom was this way before the Arizal and he only gave it a reason or if the custom developed according the the Arizal's teachings. I have never looked into that.
While the reason is not explicitly stated in the Kitzur Shulchan Aruch, it does say that you're not allowed any interruptions between putting on the shel-yad and shel-rosh. My guess is that taking the time to wrap your hand would be such an interruption.
Mishna B'rura 27:30 says to wrap the strap around your finger after putting on the head t'fila.
That said, I don't know why (which is what was asked), though I have always assumed it was so as to reduce delay.
Its source is the Arizal.
You would need a Mekubal to answer the question.
The Beis Yosef says just to wrap around bicep then put on Shel Rosh and then finsh wrapping the rest of the arm.
I have many source from the Ari, but the I'll bring one from Sha'ar HaKawanot, Tefilin Derush 5. That is the procedure the Ari brings down (first tying the first 7 and then the Shel Rosh).
It is a bit deep but the shel yad and ther shel rosh is like a choson and kallah with Hashem that's why we wrap 7 times around the arm like the kalah goes around the chosson 7 times. The middle finger is the holiest finger and that's why we put on a ring with the tefillin but you can only put on a ring when the choson and kallah are present (shel yad and shel rosh). That's why we take off the ring of tefillin before we remove the shel rosh after davening. So we don't have a ring thru a relationship with Hashem without him being present along with the shel yad and rosh. So only when both tefillin are on we can have the ring of tefillin (marriage ring) around out holy finger.
