In quite every system you need to exchange data, and the usage of a medium to move data between the trusted host and the untrusted host is _usually reasonable_ and _common practice_ (traditionally you move data from black to red with less restrictions, while the reverse procedure is more rigorous).
Making a comparison, I’d say air gapping is to networking what galvanic isolation is to circuits: you don’t have direct contact, but there’s information exchange (be it bytes or em fields).
EDIT: I would call a (strictly) isolated computer a tempest-compliant and physically isolated host.
Say you store your private PGP-key on a air-gapped computer. When receiving a sensitive document you put in on a USB-drive, enter it to the air-gapped machine and then physically destroy the USB-drive. The air-gapped machine then directly presents the decryption on a monitor.
There definitely exist scenarios where the air-gapped machine to not have to both communicate out and in, but where only in is required.
Making a comparison, I’d say air gapping is to networking what galvanic isolation is to circuits: you don’t have direct contact, but there’s information exchange (be it bytes or em fields).
EDIT: I would call a (strictly) isolated computer a tempest-compliant and physically isolated host.