there is no fundamental or 'legal' requirement for all tables to interconnect. In a relational db, I guess, its expected that there will be relations between tables. but sometimes its convenient to have one or more tables which don't relate to other tables.
I've seen "orphan" tables store things like configuration data (an EAV like table that stores miscellaenous 'stuff' needed by the application but doesn't justify its own table and isn't needed to provide data integrity checking for other tables)
I guess the real lesson is not to get hung up on the theory and go with what works for you in your design