According to the percona’s documentation, it is suggested to have recoverable backups before using the tool. Is this required, for online schema change ?
Because for osc, the tool will create a copy of the table being modified and with --nodrop-old-table option this table can act as back up. In such cases, why is the back up step mentioned in the documentation required ?
Any kind of change inherently involves some amount of risk. There could be some bug in the tool that despite all best efforts and unit tests around it that gets unearthed in your particular setup.
There could be user error where the tool works as designed but you just told it to do the wrong thing.