Upgrading JunOS

Can someone explain to like I’m 5 why do I need to use the “unlink” and “no-copy”when upgrading Juniper devices?

I know the goal is to save space, but what is the disadvantage of adding these two options with “request system software add”?

As far as I know, the unlink will not save the package in /var/tmp/install. and no-copy will not save the packages in /var/sw/pkg.

I know i can install from /var/sw/pkg/<junos-package> but what exactly the purpose of this directory and the /var/tmp/install?

thanks