It serves as a cloud backup for your Event videos.
A microSD card in Wyze Base Station lets you back up Event recordings from 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 of your cloud recordings and thumbnails.
The microSD card in your Wyze Cam Outdoor is used to store local recordings, including timelapse videos and scheduled recordings.