Why wait for some hours before filling any food into a new freezer?
Most of the "cold" in a freezer isn't stored in the air.  It's stored in the contents of the freezer, and in the walls (air has a very low volumetric specific heat).  However, the thermostat controlling the compressor works off the air temperature.

If you start filling a freezer before it's had a chance to properly cool the walls down, the food is exposed to an effective temperature that is far higher than the freezer's setpoint: heat gets transferred from the walls to the food, warming it up to unsafe levels.  Only once the walls reach thermal equilibrium with the air is it safe to start filling the freezer.