Does one really need AVR for a PC?
Yes. At least they are highly recommended to ensure maximum stability at the very least.
and at less than full load would probably be happy with less than 100V
Well, this make zero technical sense at all so there's no point in commenting on it.
Most supplies are happy with 100 - 240V so one does not need to compensate for brownouts
And this makes almost no sense. Why would you assume brownouts can't drop below the ATX stated thresholds of 90VAC (for 115VAC nominal grids) or 180VAC (for 230VAC nominal grids)? That is silly. It is also silly to assume grid voltages can't exceed the maximum limits of 135VAC and 265VAC respectively.
Plus, if the supply is expecting 115VAC and is only getting 100VAC, it still has to compensate in order to maintain the required DC voltages. Long term events like that
WILL cause excessive strain and generate more heat - never good.
What about dips (opposite of spikes) and sags (opposite of surges)? These events happen just as frequently as surges and spikes. When excessive they also put strain on the supplies and can cause the computer to crash. Any computer crash can result in corrupt data.
Note that no surge and spike protector can compensate for those low voltage anomalies. This is why surge and spike protectors are little more than fancy and expensive extension cords.
Plus, a UPS can protect more than just the PC. It can also protect the monitor(s) and the network gear as well.
Also, it is important to understand AVR works almost instantly - as in microseconds even. Without AVR, a basic backup power UPS has to sense loss of AC input and then cut-over to battery power. That can easily take 10 milliseconds or even longer. If that lapsed time exceeds just 17ms, even the most compliant power supply will drop output causing the computer to crash. And note 17ms is much faster than the human eye can detect (in lights flickering, for example). This means your computer could crash without you, as a human, even being aware there was a power sag.
Last, don't forget that destructive anomalies don't always originate from the grid. A faulty 1500W hair dryer, microwave oven, toaster, AC cycling on and off, or any other high wattage device can inject destructive anomalies too.
*****
Personal and true anecdotal experience:
Back in May 2017, I came home to hear all my UPSs beeping (I have a UPS on each computer, my home theater/TV system, even my garage door opener). Yet the house lights were still on! ???
A quick check on the LCD status display panel of my UPS showed the incoming voltage was 143VAC!!!! It should have been ~120VAC. A quick check with my multimeter confirmed, 142.8VAC and the 220VAC outlet in the garage was showing 285.6VAC! Not good - especially for things like air conditioning and refrigerator compressors.
I called the power company, told them I was a technician and what I had found and [happily] and they came right out - like in 15 minutes! The electrician checked the power entering my meter and confirmed what I had reported. This also established the problem was not on my end, but with theirs - thus affecting at least every home supplied by our common transformer.
They expedited a bucket truck out to check the transformer on the pole behind my house. They determined the transformer "tap" was faulty. They had to kill power to the whole neighborhood and move the tap temporarily until they could replace the 30 year old transformer.
They moved to a functional tap and the setting was still a little too "hot" at 258VAC (129VAC single phase). But at least that was a safe level and all the UPS (and I) were happy.
About 2 weeks later they replaced the old transformer with a brand new one and today we are sitting at 119VAC.
Had I not had an UPS on my computer I probably would have fired up the computer and with the voltage being that hot, might have put too much strain on the PSU regulator circuits. This could have resulted in several unexplained failures of the electronics in my house (and all the neighbors' houses) too. The LCD panel made troubleshooting a snap. It also allowed me to easily explain to the power company trouble desk what was going on so they sent someone out so quickly.