Why does product still get lost when there is a generator? Because a generator protects only the board it feeds; the commonest case is lighting and the office being connected while the refrigeration plant is not on the board, and nobody notices until the day of the cut.
The second reason is starting power: a compressor draws several times its running current at start-up, and a generator sized to running load cannot start it or trips. The third is pick-up time; how many seconds automatic changeover takes, or who reaches the site and how fast at night or at a weekend with a manual start, needs trying once a year. A generator left unrun for a long time also fails on battery and fuel.
The gap between the cut and pick-up may be short, but it is not zero; if a door stood open, the internal temperature rises in that time. An insulated volume with frozen plates is a backup that needs no power and protects the most sensitive batch until the generator picks up, and afterwards if it does not.