use of backup database read only dbspace

Hello,

Using IQ 16.0 SP02 with VLDB option, I'm trying to figure out which is the purpose of using:

BACKUP DATABASE READONLY DBSPACES <xx>

and in which case it may apply to do such thing ..? (simplex , MPX, backing up a parte of the db )

Usually we use Backup full and the Incremental and manual just says"these BACKUP commands specify read-only files and dbspaces"

Thank you

Regards

Jose