If you want to build ports on a machine, you should add a large (several dozen GB at least) /usr/ports partition. Keep in mind that monsters like firefox and libreoffice have insane space requirements.

Diverting ports builds to /home is a hack that works around the wrong choice made during install (which will invariably happen when you first start out, that's ok -- be prepared to reinstall with better parameters once you learn more about what you need).