M108 command requires
EMERGENCY_PARSER for full effectiveness. (Otherwise a full queue blocks the parser.)
Some G-code commands cause Marlin to go into a closed loop, waiting indefinitely for a certain state or event. For example,
M109 waits for the target temperature to be reached, and
M0 waits for an LCD click.
M108 as a "Continue" button in your host software.
M0 You're up, mate ; in your G-code fileM108 ; as your "Continue" button