Just wondering under what circumstance would someone copy the package(s) into the spool directory (using pkgadd -s ) and then actually install the packages at later time?

Does sysadmin usually copy all the common softwares into spool directory when they setup a solaris box? I mean I could just leave the dvd-rom in the tray and later simply do `pkgadd -d <source_directory> pkgname` whenever they need to add any software right?

I just don't see a point to run the pkgadd command twice to install any package. In addition, it take up double amount of disk space ( though disk space is not that expensive this day)

Not that much circumstances. The idea is to have a standard convenient place to copy some packages when if you plan to install them at a later time. The media and its drive would be available for anything else.