2-way refrigerator operation without a battery

Why does it need 12v input from a battery to operate?

http://web.archive.org/web/20200228080754/http://parallaxpower.com/wp-content/uploads/sites/8194/2018/09/Triad-Utrad-TU-models-Generic.pdf

Edit--found this for the 900 (and 700 I think)

Q. What happens when the battery is removed?
A. When the battery is removed from the vehicle, you will experience dim lights and possibly a humming
noise coming from your radio. Parallax Power Components L.L.C. does not recommend operation without
a battery connected to the system. If connected to 120 VAC at all times. Parallax Power Components
L.L.C.’s model SB100 simulated battery may be installed in place of the vehicle battery