Often, available memory on the PC is not enough for a task involving recovering hundreds of million files
— a so-called System. OutOfMemory error. The purge selected setting is just for such cases.
The button becomes active when available memory on the PC runs out.
It can be due to the fact that memory was not enough on the PC originally or just too many files
(tens or even hundreds of millions) involved.
Once the memory runs out, you get a warning message informing you
that the PC doesn’t have enough memory to continue the recovery.
You have two options: purge selected files to free up memory or continue scanning.
If you choose to purge selected, the recovery is paused and you can save the files found and then purge them.
Otherwise, the recovery is resumed and the only hope is on the Microsoft Garbage collector,
which can still free some memory on its own.