It serves as a backup for your Event videos.
A microSD card in the Base Station lets you back up Event recordings from the Wyze Cam Outdoor to the Base Station. These are the same recordings that are stored in the cloud, but the microSD serves as an additional, physical backup.
Please note: This is only for standard 12-second events. If the camera has Cam Plus enabled, events beyond 12-seconds will NOT be backed up to the Base Station SD card.