EROFS aims to form a generic read-only file system solution for various read-only use cases (embedded devices, containers and more) instead of just focusing on storage space saving without considering any side effects of runtime performance. [1]
added to Linux 5.4
adding support for file-backed mounts so that file-system image files can be used directly without needing a loopback block device or similar. This file-backed mount support is intended for use with container images and sandboxes. Using file-backed mounts is intended to avoid loopback devices that can behave intricately and can simplify error-prone lifetime management of virtual block devices.
TLDR mount an image from the FS without having to make some brand new thing in memory.[2]