Setting the HPA (immediately after secure erase) ensures that some LBAs always remain unallocated and should give the GC more candidate erase blocks to work with, increasing random write performance. Or at least that's how I understand it.
However, there already is a certain amount of free space allocated for this task. On the Intel drives, it's the difference between 80GiB and 80GB; the latter is the maximum addressable by the OS, the former is the actual capacity of the flash chips. Flash chips always have a power-of-2 capacity, and there are 5, 10 or 20 chips in the various intel drives.
Setting the HPA (immediately after secure erase) ensures that some LBAs always remain unallocated and should give the GC more candidate erase blocks to work with, increasing random write performance. Or at least that's how I understand it.