Immich actually organizes your photos in a simple directory structure, which is customizable (want to group by year? year+month? by day? not at all?). The images are right there in your file system and have the original file name.
The directory is “read-only” though, for the same reason as there is a need to import existing libraries: database synchronization.
Immich offers many features that require a database or pre-processing of files, which makes it fast and feature-rich. If you modify files outside of Immich, it cannot know what changed and loses track of where your media is.

As I said, the (read-only) file structure is always there in case you want to switch.