There are programs in ports that need curses, and so have dependencies on libcurses when being built. Those ports have curses as a dependency, and so will bring it in automatically when building. However, it’s possible that the version of libcurses already in the system will get linked instead of the ports version, so the ports software getting built could end up with the wrong library versions installed.

This walls off the system versions of various libraries, so that what gets built and linked with ports is the known, correct versions from ports. The same goes for system stuff – it will never get accidentally linked with a ports version of something and then break when that port is upgraded.