2017 Jeep Grand Cherokee, "Auxiliary" Battery's Purpose?

Hi,

    In April this year, I bought a 2017 Jeep Grand Cherokee Trail Hawk.

    Recently, I had an issue starting the vehicle on a cold morning. I bought
a new main battery and I charged the "auxiliary" (small) 12V battery. Now the
Jeep starts normally.

     There is NO information in the owner's manual about this "auxiliary"
battery.  I called the dealer where I bought it, but I did not get a "clear"
answer.  A Google search did NOT provide a GOOD answer.

      There is a REASON for this "auxiliary" (mystery) battery.  Will someone
tell me the answer to this question?

       What is the purpose of this "auxiliary" battery?

              Thank You in advance,  John