I haven't run into this on macOS yet, but on iOS the space is not reclaimed in time to avoid running out of space even with Apple Intelligence turned off:

- Not enough space to update iOS without a computer tether, even though there would have been if that ~13GB on my phone was actually freed up. Instead I had to delete several GB on my phone.

- Recordings stopping due to lack of space, and again the 13GB LLM model on my phone was not removed before running into this.

I'm actually not where if/when I'll ever get my space back, and this is on iOS 26. It takes up even more space on iOS 27, and I don't have enough free space to waste on my 256GB phone.

In practice the difference is whether the code that does this reclaiming is actually working. I’ve never gotten my Mac into a low disk space situation (other than maybe in Leopard days) but on iOS, it is quite frequent for me to trigger a situation where the free space including purgeable space is many gigabytes but actual free space is nil. Trying to commit a transaction in a SQLite database in an app? Sorry you can’t, there’s no space left on device. Now go to settings and observe that you have 15 GB left (which includes purgeable space). Even SpringBoard has trouble showing icons. I do not trust Apple’s OS level disk space purging at all. I trust the filesystem and the storage medium more.

> Philosophically, what is the difference between empty space and space that will be reclaimed when you need it?

It makes large allocations fail which would have otherwise succeeded.

"When you need it" is a state you can only enter into, AFAICT, when you approach the limit gradually, not abruptly.

Realistically, the 'claim space' service needs to run when the OS 'thinks' it has run out of space to reclaim it. And there can be M amount of waiting time for this and N number of issues that may arrive with it.