On Linux (especially on Arch Linux) instead of downloading AppImage you can compile and use your own FS2 Open engine. For example, I'm using fs2_open-git AUR package that has next files structure:

$ pacman -Ql fs2_open-git 
fs2_open-git /opt/
fs2_open-git /opt/fs2_open/
fs2_open-git /opt/fs2_open/fs2_open_24_3_0_x64_AVX2
fs2_open-git /usr/
fs2_open-git /usr/share/
fs2_open-git /usr/share/licenses/
fs2_open-git /usr/share/licenses/fs2_open-git/
fs2_open-git /usr/share/licenses/fs2_open-git/LICENSE

Trying to add this executable to a custom build will lead to error No valid or usable files were found in this build even though after linking executable path to release build it's being recognized as x86 AVX 2 exetuable.

Only workaround for now is to manually change FSO Build executable for every game under Play section.

Q: Usecase?
A:

1. Locally built executables could be compiled with machine-specific compiler optimizations that could lead to better performance.
2. When running in sandboxed environment AppImages might not have access to /tmp directory or mount system call which makes them unusable.