Sql-server – Why is there an option to rebuild/reorganize an index when SQL Server does this automatically

The operating system returned error 23(Data error (cyclic redundancy check).) to SQL Server during a read

This indicates the operating system detected an issue reading data from the hard disk itself. When data is written onto the filesystem, the operating system calculates a CRC code for each block written; when that data is subsequently read back the operating system performs the CRC operation again on the read data and compares it with the CRC stored in the filesystem. If these two CRC codes don't match the OS reports error 23. I'd be very surprised if there isn't a hardware issue with either the drive itself, or perhaps some other component such as the motherboard or drive controller.

WITH (ONLINE=ON) in the Enterprise Edition of SQL Server will have no effect whatsoever on this issue. Online index operations vs offline index operations are no different at the operating system layer; data is read and written as necessary to either rebuild or recreate the index.

That doesn't make much sense. If you restore a database from a time earlier, you won't be capturing data that has changed since the backup. (but I'm assuming you mean a backup directly followed by a restore)

If you're talking about taking a full backup and then immediately a restore, then you will persist the fragmentation in the restored database as it was when it was backed up.

So either way I will say no that is not advisable. Prudent index rebuilds/reorganization based on the fragmentation is best practice.

See this blog post that on database restoration and index fragmentation that debunks that myth. Restoring a database also restores the fragmentation at the time of the backup.