MySQL, repair a non-broken table
Repairing a table that is not crashed shouldn't break anything in the ideal case. Unfortunately, the real world is not an ideal case; a power outage or a hardware failure during the repair can leave the table in an inconsistent state. That's why backing up the table before repairing it is strongly recommended.

Also having the database offline during the maintenance operations helps fixing it faster.

Please take a look at the documentation of REPAIR TABLE for more information.