What Wanda has said here is all true.  But, there is one tidbit that she did
not mention.

If you collocate by filespace the parallelism can be attained at even
further.  But, this actually does not make sense unless the filespaces are
large; otherwise, they waste a lot of tape unless you just have small tapes.
The prime example of this is SAP backups.  But, it works for file system
backups as well.  There are always tradeoffs.