The user SFlash is a small piece of memory and consists of 4 rows. This is usually accessed by Flash write APIs and written to separately. You can see more information in the following links:
Is there any specific reason for trying to include SFlash to EEPROM emulation?
Main reason would be for simplicity. I assume each row is 512 bytes and is handled the same as application or aux flash in terms of the routines to handle those areas?