One thing to remember: packages are built using the ports tree. See the package target in the
ports(8) man page.
This means, you can use a single system with the ports tree to build packages that can be installed on other systems. This really comes in handy when you have a bunch of similar systems (build once, install many times), or when you have a fast system and a slow system (build on the fast one, install on the second one).
In effect, you can have the best of both worlds.