A UPS is temporary and 'dirty' power during a blackout. Blackouts and brownouts do not damage any properly designed harmware. A UPS provides power so that data can be saved.
A UPS can also be some of the 'dirtiest' power seen by electronics. For example, this 120 volt UPS outputs 200 volt square waves with a spike of up to 270 volts. All electronics are so robust as to consider ideal even that 'dirty' power.
Read surge protection spec numbers for a UPS. Often hundreds of joules. Destructive surges can be hundreds of thousands of joules. A UPS is near zero protection. But just enough above zero so that many will speculate a UPS does useful protection.
Blackouts will not corrupt data on any disk drive - except if using an obsolete file system. How does a disk drive first learn of a power off? DC voltages start dropping. Normal shutdown and sudden power loss is same to every disk drive. But an obsolete file system (pre-1990 vintage) on a disk can cause files to be erased.
UPS is protection of data not yet saved to the disk. And if someone does not want to wait for a reboot. It does not protect data on the drive and does not protect hardware.
Voltage fluxuations: normal voltage for any computer is even when incandescent bulbs dim to 40% intensity. How often does your AC voltage fluxuate that much? Voltage that low is good voltage for any computer. If voltage drops lower, then a computer simply powers off. Low voltage does not cause electronics damage despite urban myths that say otherwise. Urban myths exist because claims are subjective; provided without the always required hard numbers.