It is mentioned on the project page here: https://github.com/Antynea/grub-btrfs?tab=readme-ov-file#customization-of-the-grub-btrfsd-daemon
inotify-tools is required for using the grub-btrfsd daemon, not for using grub-btrfs. For this reason, they made it an optional dependency for the package:
❯ pacman -Si grub-btrfs
Repository : extra
Name : grub-btrfs
Version : 4.13-1
Description : Include btrfs snapshots in GRUB boot options
Architecture : any
URL : https://github.com/Antynea/grub-btrfs
[...]
Depends On : btrfs-progs grub
Optional Deps : snapper: For snapper support
inotify-tools: For grub-btrfsd daemon
[...]
They did it this way so folks who want to use grub-btrfs without running the daemon (for example, because they have a different solution in place for this) would not need to pull in an unneeded library.