This is necessary to prevent split blocks.

This phenomenon called split blocks occurs when during hot backup the blocks gets updated on disks. The copy in the backup file could be useless since the front and the back halves of a block may be written at different times.

To prevent this Oracle logs the before image of a data block to the redo log file before the first change, so that it can be used to reconstruct a fractured block during recovery.

The alter tablespace… begin backup command will begin logging entire block images on the first change to the block if the INIT.ORA parameter _LOGBLOCKS_DURING_BACKUP is set to true (which is the default value).

This is necessary to recover from spilt blocks.

HTH