Do you need to back up the online redo logs?
No; you never need to back up the online redo logs as part of any type of backup.
Then, why do DBAs back up the online redo logs as part of a cold backup?
One reason is that it makes the restore process for the noarchivelog mode scenario slightly easier. The online redo logs are required to open the database in a normal manner.
PURPOSE OF OPEN RESETLOGS
Sometimes, you’re required to open your database with the OPEN RESETLOGS clause.
You may do this when recreating a control file, performing a restore and recovery with a backup control file, or performing an incomplete recovery.
When you open your database with the OPEN RESETLOGS clause, it either wipes out any existing online redo log files or, if the files don’t exist, recreates them.
You can query the MEMBER column of V$LOGFILE to see which files are involved in an OPEN RESETLOGS operation.
Why would you want to wipe out what’s in the online redo logs?
Take the example of an incomplete recovery, in which the database is deliberately opened to a point in time in the past. In this situation the SCN information in the online redo logs contains transaction data that will never be recovered. Oracle forces you to open the database with OPEN RESETLOGS to purposely wipe out that information.
When you open your database with OPEN RESETLOGS, you create a new incarnation of your database and reset the log sequence number back to 1. Oracle requires a new incarnation so as to avoid accidentally using any old archive redo logs (associated with a separate incarnation of the database), in the event that another restore and recovery is required.