VRLA stands for Valve Regulated Lead Acid – for all intents and purposes a sealed battery.
VRLA batteries use an internal recombination process to absorb the gases liberated during charging. This process has limitations, so the charge voltage must be closely controlled, and varied with temperature.
VRLA batteries are irreversibly damaged by deep discharge.
A flooded battery can accept a wider range of charge voltages, but vent explosive gases, making them unsuitable for many applications.
A battery backed psu separates the battery from the load.
This means the battery is charged independently, and can be continuously monitored during this process. This allows battery faults to be indicated before the ac mains fails – increasing system reliability.
Also, separating the load and battery allows discharge protection, essential for VRLA batteries.