Eh? 70v PA is really common stuff and has been for a very long time. You probably hear it every time you're in a department store, amusement park, stadium, or other public space.
There's no need to LOL. It's useful tech.
By using a (~nominal maximum) 70v intermediate voltage, low-impedance PA systems become high-impedance. Compared to low-impedance systems at any power level, current through the wire feeding that system is reduced. Increasing impedance reduces the size of the copper wire required, and makes things easier for practical amplifiers to drive.
This is Really Useful in systems that may have dozens or hundreds of speakers distributed over a wide area.
Just to pick an example: Let's say we have 24 8-Ohm speakers to drive in a place like a grocery store. The system never needs to get proper-loud, so we'll budget 1 Watt for each speaker (24 Watts total).
If they're wired in parallel[1], then that's an impedance of 0.33 Ohms. Most power amplifiers can't drive that kind of impedance. It's also 8.5A of current, which isn't too daunting but is significant.
But if we add transformers and run things at 70v? Things get a lot easier.
Let's say we pick a 50 Watt amplifier just so we get some headroom instead of maybe running it at 100%, and that this amplifier's output power is rated at 8 Ohms.
That amplifier produces a maximum output of about 20v. Suppose we step that up to ~70V with a 1:4 transformer (which actually gives us a maximum of 80V, but again: headroom), and drive our grocery store full of 24 1-Watt 70v speakers with it.
With the same 24 Watts, our current on the speaker line drops from 8.5A to ~0.34A -- it goes from significant, to very nearly unimportant.
Our amplifier is happy: Rather than 0.33 Ohms, it sees an impedance of 12.75 Ohms.
That's an amplifier that runs cool and quiet, probably for decades. The wire can be small (18AWG seems common-enough in the grocery store overhead PA systems I've worked on; much smaller than that, and things become harder to deal with inside of ceilings). The speakers, which are all simply wired in parallel, can be replaced, removed, or added -- and also be individually adjusted in output level by changing transformer taps -- as time moves on without thinking much about the greater system architecture.
That's a lot of words, but in practice: 70v is easy. It works. We use this technique all over the globe: Some countries use 100v, and both 140v and 25v also exist as standards, but using transformers in PA-world is ridiculously common.
The usual method doesn't involve much thought at all: Add up the total power of the 70v speakers in Watts, pick an appropriate 70v amp that can drive at least that number of Watts, and send it. It's dead-simple to get right.
70v is not common in stage or musical instrument use, or in home hifi, but it doesn't have to be. It solves problems that don't exist in those spaces.
(And yeah, our grocery store needs a ~25:1 transformer on each 8 Ohm speaker. That's fine. 70v speakers that are intended to mount overhead and that include transformers with multiple secondary taps are very, very common and are still made every day in places like Chicago and Dayton[2] -- and again, you hear them all the time when you're out in public.)
[1]: We could use series-parallel arrangements to get around this, but those wiring configurations turn both arduous and easy to screw up, and loudspeakers have non-linear impedances so mixing-and-matching them in series as time moves on becomes problematic. Series-parallel isn't broadly practical.
[2]: Chicago: https://quamspeakers.com/product-group/ceiling-loudspeakers Dayton: https://fourjay.com/background-speakers/