systemd-machine-id-commit.service is an early boot service responsible for committing transient /etc/machine-id files to a writable disk file system. See machine-id(5) for more information about machine IDs.
This service is started after local-fs.target in case /etc/machine-id is a mount point of its own (usually from a memory file system such as "tmpfs") and /etc is writable. The service will invoke systemd-machine-id-setup --commit, which writes the current transient machine ID to disk and unmount the /etc/machine-id file in a race-free manner to ensure that file is always valid and accessible for other processes. See systemd-machine-id-setup(1) for details.
The main use case of this service are systems where /etc/machine-id is read-only and initially not initialized. In this case, the system manager will generate a transient machine ID file on a memory file system, and mount it over /etc/machine-id, during the early boot phase. This service is then invoked in a later boot phase, as soon as /etc/ has been remounted writable and the ID may thus be committed to disk to make it permanent.