For this question:
"Is there any advantage of backing up FRA in standby except if there is a role reversal that happens?"
The main advantage would be you don't have to do anything if anything happens to your Primary side.
For this question
" How these datafile copies and incr. backup set is created in FRA of standby too? Is that by just archivelogs being applied?"
No, applied does not do this. I would look for a job that moves a copy using scp or rsync
>Is there any advantage of backing up FRA in standby
If the standby storage / database gets corrupted you have two options
a. Take a backup from primary, restore to standby, apply archivelogs
b. Restore the standby backup, catch up the standby with the primary, apply archivelogs
Depending on the database size, the network bandwidth, frequency of database backups at standby, speed of restore, archivelog volume, speed of copying archivelogs ..... one of the two options is faster than the other. I've been at a site where option b. is faster.
Hemant K Chitale