I remember someone I knew that insisted on using a ram disk as his scratch disk...

Like, uh.... ok, you do that, good luck.

In the late 80's/early 90's, it was one of the few ways to run large videos (for the time) without stuttering bc hard drives were too slow.

There was a time when the OS could not use the whole RAM directly the hardware had, and it was a use case to put a ram disk or a swap file there.

If the OS can't see the RAM, then the OS provided ramdisk can't use it either.

But in this case, the use-case for using a ramdisk for a PS scratch disk came from using an A23-aware patched version of System 5.x/6.x with a version of Photoshop that was not A23 aware. AFAIK This was a VERY tiny window around PS1.0 and possibly PS2.0, by the time of PowerPC and System 7.5+ (which this user had), there was very definitely absolutely no advantage to partitioning the RAM away from programs.

At a guess, someone had told them to do it this way in the era of the early Mac IIs and such, but they hadn't grasped the 'why'

Was it compressed?

https://en.wikipedia.org/wiki/Zram

If the app was from an era where RAM was much smaller, but now there's enough RAM for the use case, this makes sense.

Linux is full of stuff that writes to disk all the time.

Browsers do it too. It's possible to do multiple GB an hour doing nothing on some versions with extensions, just from writing and rewriting the same file every minute for probably no good reason other than a dev hated complexity too much to check for unnecessary writes and not wear out the SSD.