@Chris_Atkinson is correct - only that usually it is only used as a Log Storage (that because it can be much larger than internal storage, especially with older models). Autoconfig files and firmware can be deployed from USB very easily, so SD was only an alternative for these tasks. Following sk111586: SD Card support for SMB Appliances, Deployment/Upgrade with SD card is only supported for the 1200R and 1400 appliances, not your 1570.
CCSP - CCSE / CCTE / CTPS / CCME / CCSM Elite / SMB Specialist